Combining native and ObjC swift_class_getInstanceExtents entry points

In the runtime currently, there are two entry points for asking for the size of a class's instances, one for known-native Swift classes, and one for possibly-ObjC classes. It looks like they're only currently used for _debugPrecondition checks in ManagedBuffer's initializers to catch improperly-sized subclasses in debug builds, so I'm wondering whether saving an `isObjC` branch in the known-native case is worth the extra entry point. Any concerns with merging them?