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

[via cfe-commits]

devajithvs wrote:

If a null-check is not ideal. I'm ok with reverting this patch.

The downside is that `clang-repl` wouldn't print the actual type. We might also need to revert this: https://github.com/llvm/llvm-project/pull/187725, which is similar to what this patch does.

I also found that we can trigger the assertion also with `UnaryTransformType`, and I think this might not be the last. Maybe we should replace the assert with a safe fallback regardless of what we do?

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