Support operator keywords used in Windows SDK(fix ubsan)

UBSan found an issue with a nullptr being assigned to a reference.
This was because a following function went back and checked the 
identifier in the CPPOperatorName case.  This patch corrects that
location with the original logic as well.

llvm-svn: 305128

clang/lib/Lex/Preprocessor.cpp

@@ -713,7 +713,9 @@ bool Preprocessor::HandleIdentifier(Token &Identifier) {
   // C++ 2.11p2: If this is an alternative representation of a C++ operator,
   // then we act as if it is the actual operator and not the textual
   // representation of it.
-  if (II.isCPlusPlusOperatorKeyword())
+  if (II.isCPlusPlusOperatorKeyword() &&
+      !(getLangOpts().MSVCCompat &&
+        getSourceManager().isInSystemHeader(Identifier.getLocation())))
     Identifier.setIdentifierInfo(nullptr);
 
   // If this is an extension token, diagnose its use.