relates to:

• SR-11268 Builtins for objc_msgSend/objc_msgSend_super

Issue Description:

The standard library has a trick it uses where it declares an @objc protocol with some selectors, and then unsafeBitCasts things to it to get swiftc to generate objc_msgSends without having to link the libraries that declare those methods.

This works great up until you attempt to get ARC to stop defensively retaining the receiver:

* Putting @_effects(releasenone) on the function that does the bit cast doesn't work because the bit cast loses the object identity

* Putting @_effects(releasenone) on the protocol doesn't work, presumably because protocols don't support that

* Putting @_effects(releasenone) on a second "thunk" function that takes the protocol type does work, and is what I'm doing in #26422 but is pretty clunky

Could we have a nicer way to do this? A few possible ideas, which I have no preference between:

* Could make @_effects work on protocol declarations

* Could teach ARC that unsafeBitCasting from a reference type to an @objc protocol type maintains object identity and maintains @_effects relevant to that identity

* Could provide a builtin or something for calling objc methods that the compiler doesn't statically know about, instead of unsafeBitCasting to a fake protocol