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

[Max Winkler via cfe-commits]

https://github.com/MaxEW707 approved this pull request.

LGTM!

@xbjfk Let us know if you require one of us to commit on your behalf due to not having commit access.

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