[clang] [libcxx] [Clang][POC] Atomic operations on _BitInt(N) (PR #204815)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

I do think we should also test something in C23 and C2y modes.

BTW, is there anything disallowing `_Atomic _BitInt(N)` in C23 (or latest C2y draft)?

WG14 [N2763](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2763.pdf) said that

> Removed the _BitwidthOf, file IO, literal suffixes, and atomic functionality and added a Future Directions section to the prose

But it seems that C23 still requires support for `_Atomic _BitInt(N)`...

https://github.com/llvm/llvm-project/pull/204815