We attempted to use the UnaryTransformType's UnderlyingType instead of
it's BaseType.  This is not correct for dependent UnaryTransformType
because the have no underlying type.

This fixes PR28045.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272079 91177308-0d34-0410-b5e6-96231b3b80d8