[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 think this should be fine at this point. I’m having a bit of a hard time trying to figure out what changed since my last review because of the force-pushing, but so long as you haven’t made any other major changes since then then this ltgm now.

Sorry about that. This PR had become so stale that there merge conflicts and the easiest way to fix it was to squash because the conflicts were with things I later removed.

Thanks for reviewing and approving. I'm going to cherry-pick this into the Swiftlang fork of llvm-project first and fix everything up (there's stuff that wasn't upstreamed that's going to conflict with this PR). Once I've done that I'll merge this.

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