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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I did a pass, I think @Sirraide is on the right direction in his reviews here, but the general direction of this patch is also completely acceptable.

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