[NFC] Fixup TypeChecker::getOptionalType To Not Return Type()

lib/Sema/CSSimplify.cpp

@@ -6889,6 +6889,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
                                         TVO_CanBindToLValue |
                                         TVO_CanBindToNoEscape);
       Type optTy = TypeChecker::getOptionalType(SourceLoc(), innerTV);
+      assert(!optTy->hasError());
       SmallVector<Constraint *, 2> optionalities;
       auto nonoptionalResult = Constraint::createFixed(
           *this, ConstraintKind::Bind,