Rename LangOptions::Microsoft to LangOptions::MicrosoftExt to make it clear that...

Rename LangOptions::Microsoft to LangOptions::MicrosoftExt to make it clear that this flag must be used only for Microsoft extensions and not emulation; to avoid confusion with the new LangOptions::MicrosoftMode flag.

Many of the code now under LangOptions::MicrosoftExt will eventually be moved under the LangOptions::MicrosoftMode flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139987 91177308-0d34-0410-b5e6-96231b3b80d8