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

[Aaron Ballman via cfe-commits]

AaronBallman 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`)
> 
> Hmmm okay, that seems reasonable enough.

I've added that now.

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