[clang] [Clang] Fix crash on malformed std::partial_ordering static members (PR #172001)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

LGTM, just some super minor nits on style, and I'd just like a few more tests - you've added logic to be safe against `using partial_ordering = int`, etc so we should make sure that's tested.

https://github.com/llvm/llvm-project/pull/172001