[ConstraintSystem] Resolve dependent member as a hole if base is one

Instead of preserving unresolved dependent member types, let's resolve
them as holes if their base has been determined to be one.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -4274,8 +4274,7 @@ bool ConstraintSystem::repairFailures(
     // it's going to be diagnosed by specialized fix which deals
     // with generic argument mismatches.
     if (matchKind == ConstraintKind::BindToPointerType) {
-      auto *member = rhs->getAs<DependentMemberType>();
-      if (!(member && member->getBase()->hasPlaceholder()))
+      if (!rhs->isPlaceholder())
         break;
     }
 

lib/Sema/ConstraintSystem.cpp

@@ -2958,6 +2958,10 @@ Type ConstraintSystem::simplifyTypeImpl(Type type,
       // Simplify the base.
       Type newBase = simplifyTypeImpl(depMemTy->getBase(), getFixedTypeFn);
 
+      if (newBase->isPlaceholder()) {
+        return PlaceholderType::get(getASTContext(), depMemTy);
+      }
+
       // If nothing changed, we're done.
       if (newBase.getPointer() == depMemTy->getBase().getPointer())
         return type;