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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman
> 
> I'm not sure who's responsible for `<stdcountof.h>` in LLVM.

Yeah, it's never really clear who is expected to provide these headers.

> 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.

I think Clang should probably provide the header as well. We do so for stdalign.h, stdbool.h, stdnoreturn.h and this is morally equivalent to those. Thanks for bringing this up!

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