Sema: Fix yet another crash when accessing nested types of an existential

We diagnose usage of invalid existential types but we might still
try to compute substitutions. Just whip up an ErrorType instead.

Fixes <rdar://problem/16803384>.

Swift SVN r29567

lib/Sema/ConstraintSystem.cpp

@@ -1082,6 +1082,13 @@ ConstraintSystem::getTypeOfMemberReference(Type baseTy, ValueDecl *value,
   // Handle associated type lookup as a special case, horribly.
   // FIXME: This is an awful hack.
   if (auto assocType = dyn_cast<AssociatedTypeDecl>(value)) {
+    // Error recovery path.
+    if (baseObjTy->isOpenedExistential()) {
+      Type memberTy = ErrorType::get(TC.Context);
+      auto openedType = FunctionType::get(baseObjTy, memberTy);
+      return { openedType, memberTy };
+    }
+
     // Refer to a member of the archetype directly.
     if (auto archetype = baseObjTy->getAs<ArchetypeType>()) {
       Type memberTy = archetype->getNestedTypeValue(value->getName());