[MS] Raise the default value of _MSC_VER to 1911, which is VS 2017

Summary:
This raises our default past 1900, which controls whether char16_t is a
builtin type or not.

Implements PR34243

Reviewers: hans

Subscribers: STL_MSFT, rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D38646

llvm-svn: 315107

clang/docs/ReleaseNotes.rst

@@ -89,6 +89,11 @@ Non-comprehensive list of changes in this release
 - Bitrig OS was merged back into OpenBSD, so Bitrig support has been
   removed from Clang/LLVM.
 
+- The default value of _MSC_VER was raised from 1800 to 1911, making it
+  compatible with the Visual Studio 2015 and 2017 C++ standard library headers.
+  Users should generally expect this to be regularly raised to match the most
+  recently released version of the Visual C++ compiler.
+
 New Compiler Flags
 ------------------
 

clang/lib/Driver/ToolChains/MSVC.cpp

@@ -1266,9 +1266,8 @@ VersionTuple MSVCToolChain::computeMSVCVersion(const Driver *D,
   if (MSVT.empty() &&
       Args.hasFlag(options::OPT_fms_extensions, options::OPT_fno_ms_extensions,
                    IsWindowsMSVC)) {
-    // -fms-compatibility-version=18.00 is default.
-    // FIXME: Consider bumping this to 19 (MSVC2015) soon.
-    MSVT = VersionTuple(18);
+    // -fms-compatibility-version=19.11 is default, aka 2017
+    MSVT = VersionTuple(19, 11);
   }
   return MSVT;
 }