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

[Alejandro Colomar via cfe-commits]

alejandro-colomar wrote:

@AaronBallman 

I'm not sure who's responsible for `<stdcountof.h>` in LLVM.  In GCC it's a header provided by the compiler.  I see that this PR does not add the header, so I was wondering if that's to be added in a separate commit, or if it's not supported by Clang and depends on library support.

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