[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Kees Cook via cfe-commits]

https://github.com/kees approved this pull request.

I can't speak to the LLVM code changes, but behaviorally, this passes all the torture tests I'd expect it to (kernel-tools/fortify/array-bounds.c and full kernel builds with its hundreds of `counted_by` annotations).

https://github.com/llvm/llvm-project/pull/76348