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

[Alejandro Colomar via cfe-commits]

alejandro-colomar wrote:

> > Thanks!
> > For the first commit:
> > ```
> > Suggested-by: Alejandro Colomar <alx at kernel.org>
> > ```
> 
> Happy to add it,

Thanks!  :)

> but for my own education: what's that do? :-D

<https://www.kernel.org/doc/html/latest/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes>

Quoting that:

```
A Suggested-by: tag indicates that the patch idea is suggested by
the person named and ensures credit to the person for the idea:
if we diligently credit our idea reporters, they will, hopefully,
be inspired to help us again in the future. Note, this is one of
only three tags you might be able to use without explicit
permission of the person named (see ‘Tagging people requires
permission’ below for details).
```

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