[clang] [clang][AST] Fix assertion in `getFullyQualifiedType` for AutoType (PR #186105)

[Harald van Dijk via cfe-commits]

hvdijk wrote:

It's easy to make it not crash but it's less clear to figure out what the right thing to do is. My initial thought is that `getFullyQualifiedType` shouldn't resolve `auto`, because in some cases it cannot resolve `auto`, and only resolving `auto` in those cases where it can makes the API inconsistent. But that would mean reverting this PR and fixing the assert that you were working on differently, and I don't know how you feel about that.

https://github.com/llvm/llvm-project/pull/186105