[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:

@bwendling @kees Apologies for taking so long to get this change upstream. We spent quite a lot of time experimenting with different implementations that ended up not working out. Our internal version of this patch is larger because `counted_by` is allowed in many more contexts than it is in upstream Clang so some of that has been omitted from this patch. I can upstream the rest once we have support for `counted_by` is more contexts.

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