[clang] [BoundsSafety] WIP: Make 'counted_by' work for pointer fields; late parsing for 'counted_by' on decl attr position (PR #87596)

[Dan Liew via cfe-commits]

delcypher wrote:

@rapidsna I've addressed the "Make it an error to use 'counted_by' on size-less types (e.g., forward declared structs)" TODO.

I still need to test this PR properly and write a proper commit message explaining what this does.

https://github.com/llvm/llvm-project/pull/87596