[clang] [PAC][clang] Correct handling of ptrauth queries of incomplete types (PR #164528)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> These changes look correct to me (do we need a release note?)

It seems super minor, do you know of any other way I might be able to get into the state of having a record containing an incomplete state? Otherwise "fix a crash when pointer auth is enabled, and attempting to construct a templated type that has not actually been instantiated" seems a bit wordy :D

https://github.com/llvm/llvm-project/pull/164528