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

[Kees Cook via cfe-commits]

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

Thanks for the updates! Let's get this in and continue with the rest of the support. :)

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