Preprocessor: make C++ operator names as macro identifiers a compatible extension

With recent changes, this is now a compatible language extension and can be
safely enabled with -ms-extensions instead of requiring the full
-ms-compatibility MSVC drop-in mode. As such we can now also emit an extension
warning under -Wmicrosoft to help users port their code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209978 91177308-0d34-0410-b5e6-96231b3b80d8