[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:

I had to tweak some minor issues with the diagnostic text that I discovered when trying to cherry pick this into our fork. I've fixed them and I'm going to land this now.

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