[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 Thanks for your review and sorry for the delay in getting back to this. I've tried to address as much as possible and left comment on things that still need discussion. Please take a look when time permits.

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