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

[Yeoul Na via cfe-commits]

rapidsna wrote:

@bwendling @kees Likely, we should not put `__counted_by` in that case. Could we fix the source?

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