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

[Harald van Dijk via cfe-commits]

hvdijk wrote:

For a dependent `auto`, `AT->isDeduced()` is true but `AT->getDeducedType().isNull()` is also true and if that case is hit, the subsequent `QT.getTypePtr()` segfaults. I'm not sure if it's possible to hit this in `clang-repl` but it's definitely possible in other code using Clang as a library (I'm seeing the segfault in Shiboken), and I'm not sure what the expected result of `getFullyQualifiedType` would be in that case.

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