Implement filesystem NB comments, relative paths, and related issues.

This is a fairly large patch that implements all of the filesystem NB comments
and the relative paths changes (ex. adding weakly_canonical). These issues
and papers are all interrelated so their implementation couldn't be split up
nicely.

This patch upgrades <experimental/filesystem> to match the C++17 spec and not
the published experimental TS spec. Some of the changes in this patch are both
API and ABI breaking, however libc++ makes no guarantee about stability for
experimental implementations.

The major changes in this patch are:

* Implement NB comments for filesystem (P0492R2), including:
  * Implement `perm_options` enum as part of NB comments, and update the
    `permissions` function to match.
  * Implement changes to `remove_filename` and `replace_filename`
  * Implement changes to `path::stem()` and `path::extension()` which support
    splitting examples like `.profile`.
  * Change path iteration to return an empty path instead of '.' for trailing
    separators.
  * Change `operator/=` to handle absolute paths on the RHS.
  * Change `absolute` to no longer accept a current path argument.

* Implement relative paths according to NB comments (P0219r1)

* Combine `path.cpp` and `operations.cpp` since some path functions require
  access to the operations internals, and some fs operations require access
  to the path parser.

llvm-svn: 329028
diff --git a/libcxx/test/support/verbose_assert.h b/libcxx/test/support/verbose_assert.h
new file mode 100644
index 0000000..353e71c
--- /dev/null
+++ b/libcxx/test/support/verbose_assert.h
@@ -0,0 +1,222 @@
+#ifndef TEST_SUPPORT_VERBOSE_ASSERT
+#define TEST_SUPPORT_VERBOSE_ASSERT
+
+#include <iostream>
+#include <cstdio>
+#include <sstream>
+#include <string>
+#include "test_macros.h"
+
+namespace verbose_assert {
+
+typedef std::basic_ostream<char>&(EndLType)(std::basic_ostream<char>&);
+
+template <class Stream, class Tp,
+    class = decltype(std::declval<Stream&>() << std::declval<Tp const&>())>
+std::true_type IsStreamableImp(int);
+template <class Stream, class Tp> std::false_type IsStreamableImp(long);
+
+template <class Stream, class Tp>
+struct IsStreamable : decltype(IsStreamableImp<Stream, Tp>(0)) {};
+
+template <class Tp, int ST = (IsStreamable<decltype(std::cerr), Tp>::value ? 1
+        : (IsStreamable<decltype(std::wcerr), Tp>::value ? 2 : -1))>
+struct SelectStream {
+  static_assert(ST == -1, "specialization required for ST != -1");
+  static void Print(Tp const&) { std::clog << "Value Not Streamable!\n"; }
+};
+
+template <class Tp>
+struct SelectStream<Tp, 1> {
+  static void Print(Tp const& val) { std::cerr << val; }
+};
+
+template <class Tp>
+struct SelectStream<Tp, 2> {
+  static void Print(Tp const& val) { std::wcerr << val; }
+};
+
+struct AssertData {
+  AssertData(const char* xcheck, const char* xfile, const char* xfunc,
+             unsigned long xline, bool xpassed = true)
+      : passed(xpassed), check(xcheck), file(xfile), func(xfunc), line(xline),
+        msg() {}
+
+  AssertData& SetFailed(std::string xmsg = std::string()) {
+    msg = xmsg;
+    passed = false;
+    return *this;
+  }
+
+  void PrintFailed() const {
+    std::fprintf(stderr, "%s:%lu %s: Assertion '%s' failed.\n", file, line,
+                 func, check);
+    if (!msg.empty())
+      std::fprintf(stderr, "%s\n", msg.data());
+  }
+
+  bool passed;
+  const char* check;
+  const char* file;
+  const char* func;
+  unsigned long line;
+  std::string msg;
+};
+
+// AssertHandler is the class constructed by failing CHECK macros. AssertHandler
+// will log information about the failures and abort when it is destructed.
+class AssertHandler {
+public:
+  AssertHandler(AssertData const& Data)
+      : passed(Data.passed) {
+    if (!passed)
+      Data.PrintFailed();
+  }
+
+  ~AssertHandler() TEST_NOEXCEPT_FALSE {
+    if (!passed) {
+      error_log << std::endl;
+      std::abort();
+    }
+  }
+
+  class LogType {
+    friend class AssertHandler;
+
+    template <class Tp>
+    friend LogType& operator<<(LogType& log, Tp const& value) {
+      if (!log.is_disabled) {
+        SelectStream<Tp>::Print(value);
+      }
+      return log;
+    }
+
+    friend LogType& operator<<(LogType& log, EndLType* m) {
+      if (!log.is_disabled) {
+        SelectStream<EndLType*>::Print(m);
+      }
+      return log;
+    }
+
+  private:
+    LogType(bool disable) : is_disabled(disable) {}
+    bool is_disabled;
+
+    LogType(LogType const&);
+    LogType& operator=(LogType const&);
+  };
+
+  LogType& GetLog() {
+    if (passed)
+      return null_log;
+    return error_log;
+  }
+
+private:
+  static LogType null_log;
+  static LogType error_log;
+
+  AssertHandler& operator=(const AssertHandler&) = delete;
+  AssertHandler(const AssertHandler&) = delete;
+  AssertHandler() = delete;
+
+private:
+  bool passed;
+};
+
+AssertHandler::LogType AssertHandler::null_log(true);
+AssertHandler::LogType AssertHandler::error_log(false);
+
+template <class It1>
+std::string PrintRange(const char* Name, It1 F, It1 E) {
+  std::stringstream ss;
+  ss << "  " << Name << " = [";
+  while (F != E) {
+    ss << *F;
+    ++F;
+    if (F != E)
+      ss << ", ";
+  }
+  ss << "]\n";
+  return ss.str();
+}
+
+template <class Tp, class Up>
+std::string PrintMismatch(Tp const& LHS, Up const& RHS, int Elem) {
+  std::stringstream ss;
+  ss << "  Element " << Elem << " mismatched: `" << LHS << "` != `" << RHS
+     << "`!\n";
+  return ss.str();
+};
+
+struct EqualToComp {
+  template <class Tp, class Up>
+  bool operator()(Tp const& LHS, Up const& RHS) const {
+    return LHS == RHS;
+  }
+};
+
+template <class It1, class It2, class Comp>
+AssertData CheckCollectionsEqual(It1 F1, It1 E1, It2 F2, It2 E2,
+                                 AssertData Data, Comp C = EqualToComp()) {
+  const It1 F1Orig = F1;
+  const It2 F2Orig = F2;
+  bool Failed = false;
+  std::string ErrorMsg;
+  int Idx = 0;
+  while (F1 != E1 && F2 != E2) {
+    if (!(C(*F1, *F2))) {
+      ErrorMsg += PrintMismatch(*F1, *F2, Idx);
+      Failed = true;
+      break;
+    }
+    ++Idx;
+    ++F1;
+    ++F2;
+  }
+  if (!Failed && (F1 != E1 || F2 != E2)) {
+    ErrorMsg += "  Ranges have different sizes!\n";
+    Failed = true;
+  }
+  if (Failed) {
+    ErrorMsg += PrintRange("LHS", F1Orig, E1);
+    ErrorMsg += PrintRange("RHS", F2Orig, E2);
+    Data.SetFailed(ErrorMsg);
+  }
+  return Data;
+}
+} // namespace verbose_assert
+
+#ifdef __GNUC__
+#define ASSERT_FN_NAME() __PRETTY_FUNCTION__
+#else
+#define ASSERT_FN_NAME() __func__
+#endif
+
+#define DISPLAY(...) "    " #__VA_ARGS__ " = " << (__VA_ARGS__) << "\n"
+
+#define ASSERT(...)                                                            \
+  ::verbose_assert::AssertHandler(::verbose_assert::AssertData(                \
+    #__VA_ARGS__, __FILE__, ASSERT_FN_NAME(), __LINE__,(__VA_ARGS__))).GetLog()
+
+#define ASSERT_EQ(LHS, RHS) \
+  ASSERT(LHS == RHS) << DISPLAY(LHS) << DISPLAY(RHS)
+#define ASSERT_NEQ(LHS, RHS) \
+  ASSERT(LHS != RHS) << DISPLAY(LHS) << DISPLAY(RHS)
+#define ASSERT_PRED(PRED, LHS, RHS) \
+  ASSERT(PRED(LHS, RHS)) << DISPLAY(LHS) << DISPLAY(RHS)
+
+#define ASSERT_COLLECTION_EQ_COMP(F1, E1, F2, E2, Comp)                        \
+  (::verbose_assert::AssertHandler(                                            \
+       ::verbose_assert::CheckCollectionsEqual(                                \
+           F1, E1, F2, E2,                                                     \
+           ::verbose_assert::AssertData("CheckCollectionsEqual(" #F1 ", " #E1  \
+                                        ", " #F2 ", " #E2 ")",                 \
+                                        __FILE__, ASSERT_FN_NAME(), __LINE__), \
+           Comp))                                                              \
+       .GetLog())
+
+#define ASSERT_COLLECTION_EQ(F1, E1, F2, E2)                                   \
+  ASSERT_COLLECTION_EQ_COMP(F1, E1, F2, E2, ::verbose_assert::EqualToComp())
+
+#endif