[clang] [BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable (PR #106321)

[Dan Liew via cfe-commits]

delcypher wrote:

@Sirraide Apologies for taking so long to get back to this PR. I think I've addressed all your feedback now. Is this ok to land now?

https://github.com/llvm/llvm-project/pull/106321