[clang] [clang] Add /Zc:__STDC__ flag to clang-cl (PR #68690)

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xbjfk wrote:

Okay, I made sure it says `argument unused during compilation: '/Zc:__STDC__'` and never defines __STDC__ in C++ mode, but I have couple of questions:
 - Where should I put the test to make sure `-fno-ms-define-stdc` is not accepted?
 -  !IsCXX feels kind of wrong - is this a justified case for creating `isC` in `clang/lib/Driver/Types.cpp`?
    - now that I think about it, what should the behavior be with Objective-C? I imagine the number of people compiling Objective-C with `-fms-compatibility` is very small
 
Thank you 

https://github.com/llvm/llvm-project/pull/68690