[clang] [C2y] Implement WG14 N3369 and N3469 (_Countof) (PR #133125)

[via cfe-commits]

https://github.com/cor3ntin commented:

Did WG14 considered some sort of macros for this feature?
Otherwise I'd be tempted to ask you to add something to `__has_feature`. I can see user wanting to use `_Countof` when available and fallback to something else when not.

https://github.com/llvm/llvm-project/pull/133125