As per discussion with Doug Gregor on the IRC channel, introduce a new...

As per discussion with Doug Gregor on the IRC channel, introduce a new compiler switch: -fms-compatility. 

Microsoft specific tweaking will now fall into 2 categories:

    - fms-extension: Microsoft specific extensions that should never change the meaning of an otherwise well formed code. Currently map to LangOptions::Microsoft. (To be clearer, I am planning to change the name to LangOptions::MicrosoftExt).

    - fms-compatibility: Really a MSVC emulation mode. Map to LangOptions::MicrosoftMode. Can change the meaning of an otherwise standard conformant program.

llvm-svn: 139978