[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:

[This comment](https://github.com/llvm/llvm-project/pull/106321/files#r1906005324) is not showing up in the conversation view for some reason.

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