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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I found a few minor nits, but I think we're pretty close!

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