[clang] Avoid assert in substqualifier (PR #182707)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> I believe this assertion is correct - I don't believe that we should be attempting to instantiate an invalid template declaration in the first place?

Nope, we do need to instantiate for diagnostics. Hmmm.

https://github.com/llvm/llvm-project/pull/182707