[clang] [clang] __STDC_NO_THREADS__ is no longer necessary for VS 2022 1939 and above (PR #117149)

[Dipesh Sharma via cfe-commits]

dipeshs809 wrote:

> Thank you for working on this! Please be sure to add some test coverage for the changes to `clang/test/Headers` and a release note in `clang/docs/ReleaseNotes.rst`.

Thanks for the review. I'll incorporate the changes suggested and come back to you.

https://github.com/llvm/llvm-project/pull/117149