[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:

@Sirraide 's comments are all relevant, and he's complained about everything I would have.  So when he's happy with this, I am too.

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