[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 @erichkeane  Sorry for the delay in addressing your feedback. This PR is ready for another round of reviews. Note I'm aware there are some code formatting problems. I'll fix them when I squash locally. I've tried to keep all my tweaks as separate commits so it's easier to review.

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