[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:

@erichkeane @AaronBallman @Sirraide I've addressed everything I can. 

I seem to be running into a problem where some of my replies to comments are not showing up in the conversation tab. This seems to be a [known problem with GitHub reviews ](https://github.com/orgs/community/discussions/6893#discussioncomment-8309254):( . You may have to dig into the Files tab to see some of my replies to comments :(

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