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

[Alejandro Colomar via cfe-commits]

https://github.com/alejandro-colomar approved this pull request.

Thanks!

For the first commit:
```
Suggested-by: Alejandro Colomar <alx at kernel.org>
```

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