[Sema] Fix getEffectiveOverloadType to recognize when base is opaque result type

Previously only nominal type and existential where allowed but
that's outdated, it's possible to reference a member of an opaque
result type as well, the concrete type in this case is going to
be deduced by the solver.

Author: xedin

lib/Sema/TypeOfReference.cpp

@@ -1906,7 +1906,8 @@ Type ConstraintSystem::getEffectiveOverloadType(ConstraintLocator *locator,
           return Type();
 
         baseTy = baseTy->getRValueType();
-        if (!baseTy->getAnyNominal() && !baseTy->is<ExistentialType>())
+        if (!baseTy->getAnyNominal() && !baseTy->is<ExistentialType>() &&
+            !baseTy->is<OpaqueTypeArchetypeType>())
           return Type();
       }