Sema: Better fix for banning the '() -> T' => '() -> ()?' implicit conversion

Previously we would check TMF_UnwrappingOptional flag, which does not
stick with the constraint, so it would not always persist. Now, add a
new OptionalPayload locator element, which is more correct.

Fixes <rdar://problem/30429709>.

lib/Sema/CSSimplify.cpp

@@ -3874,7 +3874,8 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
       if (generic2->getDecl()->classifyAsOptionalType()) {
         return matchTypes(type1, generic2->getGenericArgs()[0],
                           matchKind, (subflags | TMF_UnwrappingOptional),
-                          locator);
+                          locator.withPathElement(
+                            ConstraintLocator::OptionalPayload));
       }
     }