[clang] [BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable (PR #106321)

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Sirraide wrote:

> I seem to be running into a problem where some of my replies to comments are not showing up in the conversation tab.

Yeah, I prefer looking at the flood of emails I get from GitHub, because those pretty consistently *do* include every comment.

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