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

[Yeoul Na via cfe-commits]

rapidsna wrote:

> The main concern I have with delaying support for this is that header files could find themselves in a state where they could not be refactored without removing counted_by attributes that refer to now-incomplete structs.

@kees Agreed. We will work on a follow up patch to support this, and a separate patch to support `sized_by` for `void *` and cases where it needs a byte size.

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