[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.
> 
> 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!

Okay, I'll see if I can write this patch.  I'm much less busy these days.  :)

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