[clang] [C2y] Add stdcountof.h (PR #140890)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I still have no problem with this, but it is now into things I have zero confidence reviewing :) Please have someone who has ever used modules review this.

I've added some modules reviewers, so hopefully they can take a look. Yeah, I agree that tablegen would be a nice thing to have here, but it's a bit odd because the header file itself cannot be table generated.

https://github.com/llvm/llvm-project/pull/140890