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/filesystem_test_helper.hpp b/libcxx/test/support/filesystem_test_helper.hpp
index 622a60d..5a8a32a 100644
--- a/libcxx/test/support/filesystem_test_helper.hpp
+++ b/libcxx/test/support/filesystem_test_helper.hpp
@@ -9,7 +9,6 @@
 #include <random>
 #include <chrono>
 
-
 // static test helpers
 
 #ifndef LIBCXX_FILESYSTEM_STATIC_TEST_ROOT
@@ -400,4 +399,8 @@
         ;
 }
 
+inline bool PathEq(fs::path const& LHS, fs::path const& RHS) {
+  return LHS.native() == RHS.native();
+}
+
 #endif /* FILESYSTEM_TEST_HELPER_HPP */