[clang] Reland #90786 ([BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C) (PR #93121)

[Dan Liew via cfe-commits]

delcypher wrote:

@rapidsna Please check 604e274a5bc37b18c3bc89eed5749c8617de36e7 to check you are happy with the way I'm fixing the memory leak.

https://github.com/llvm/llvm-project/pull/93121