This replaces the '[volatile]' flag. Now, class_method and
super_method are only used for vtable dispatch.
The witness_method instruction is still overloaded for use
with both ObjC protocol requirements and Swift protocol
requirements; the next step is to make it only mean the
latter, also using objc_method for ObjC protocol calls.
This removes the -use-native-super-method flag and turns on dynamic
dispatch for native method invocations on super by default.
rdar://problem/22749732
