Sema: Fix 'super' calls from methods returning 'Self'
We have a quirk where TypeBase::getSuperclass() on DynamicSelfType
returns the underlying class type, and not the superclass of the
underlying class type. As a result, we would emit a SuperRefExpr
whose type was the type of 'self' and not the type of 'super'.
Fixes <rdar://problem/30853768>.