[clang] [BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C (PR #90786)

[Dan Liew via cfe-commits]

delcypher wrote:

@kees Thanks for approving.

I'm going to resolve the merge conflict in `clang/docs/ReleaseNotes.rst` and then merge. I'll start looking at supporting `__counted_by()` on incomplete pointee types next. @hnrklssn is going to start working on upstreaming the `__sized_by` attribute soon.

https://github.com/llvm/llvm-project/pull/90786