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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! Please be sure to add a release note to `clang/docs/ReleaseNotes.rst` so users know about the new functionality.

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