[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]

erichkeane wrote:

I think I'm reasonably OK with this, though @Sirraide has a 'requested changes', so he should do the final approval.

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