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

[Dan Liew via cfe-commits]

delcypher wrote:

@bwendling @kees Any further feedback? If not, can you approve?

As @rapidsna said we'll follow up this PR with additional PRs to address the two major concerns you had.

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