It is possible for ClassDecl::getSuperclassDecl() to succeed but for
ClassDecl::getSuperclass() to fail. This happens if the superclass is
a generic type and one of the generic arguments could not be resolved,
or does not satisfy the generic requirements, for example; in that
case, a BoundGenericType cannot be formed.
In a couple of places we were not prepared for this possibility.
Let's recover by making judicious use of ErrorType.
Fixes <rdar://problem/73169149>.
