Use std::error_code instead of llvm::error_code.

The idea of this patch is to turn llvm/Support/system_error.h into a
transitional header that just brings in the erorr_code api to the llvm
namespace. I will remove it shortly afterwards.

The cases where the general idea needed some tweaking:

* std::errc is a namespace in msvc, so we cannot use "using std::errc". I could
add an #ifdef, but there were not that many uses, so I just added std:: to
them in this patch.

* Template specialization had to be moved to the std namespace in this
patch set already.

* The msvc implementation of default_error_condition doesn't seem to
provide the same transformations as we need. Not too surprising since
the standard doesn't actually say what "equivalent" means. I fixed the
problem by keeping our old mapping and using it at error_code
construction time.

Despite these shortcomings I think this is still a good thing. Some reasons:

* The different implementations of system_error might improve over time.
* It removes 925 lines of code from llvm already.
* It removes 6313 bytes from the text segment of the clang binary when
it is built with gcc and 2816 bytes when building with clang and
libstdc++.

llvm-svn: 210687
diff --git a/llvm/unittests/Support/ErrorOrTest.cpp b/llvm/unittests/Support/ErrorOrTest.cpp
index 7ace5e3..976a953 100644
--- a/llvm/unittests/Support/ErrorOrTest.cpp
+++ b/llvm/unittests/Support/ErrorOrTest.cpp
@@ -16,7 +16,7 @@
 namespace {
 
 ErrorOr<int> t1() {return 1;}
-ErrorOr<int> t2() { return errc::invalid_argument; }
+ErrorOr<int> t2() { return std::errc::invalid_argument; }
 
 TEST(ErrorOr, SimpleValue) {
   ErrorOr<int> a = t1();
@@ -30,7 +30,7 @@
 
   a = t2();
   EXPECT_FALSE(a);
-  EXPECT_EQ(errc::invalid_argument, a.getError());
+  EXPECT_EQ(std::errc::invalid_argument, a.getError());
 #ifdef EXPECT_DEBUG_DEATH
   EXPECT_DEBUG_DEATH(*a, "Cannot get value when an error exists");
 #endif