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

[via cfe-commits]

cor3ntin wrote:

> So you want this to be a FEATURE in C2y and an EXTENSION in older language modes? 

Presumably yes.

> And when we get the countof macro in a few months, this becomes unnecessary?

Are most users going to include `stdcountof.h` ?

My assumption is that they would not, so having the feature thingy  is useful (especially in a library that may be concerned about not defining `countof`)

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