Move the ObjC class name stability check logic to the Swift runtime, exposing it as a new SPI called _swift_isObjCTypeNameSerializable.
Update the reporting logic. The ObjC names of generic classes are considered stable now, but private classes and classes defined in function bodies or other anonymous contexts are unstable by design.
On the overlay side, rewrite the check’s implementation in Swift and considerably simplify it.
rdar://57809977
