relates to:

• SR-11249 Using @_effects(releasenone) is tricky with shadow protocols

Issue Description:

Note: this would obviate the need for https://bugs.swift.org/browse/SR-11249 entirely

The stdlib has a trick that it uses to interact with Cocoa that we've dubbed "shadow protocols". The basic idea is this:

@objc protocol Shadow {
    @objc(doThing:) func doTheThing() -> Int
}
 
@_effects(releasenone)
private func doThingImpl(_ x: Shadow) -> Int {
    return x.doTheThing()
}
 
@_effects(releasenone)
func cocoaDoThing(_ x: AnyObject /* but we know it's actually a particular ObjC object type */) -> Int {
    return doThingImpl(_unsafeReferenceCast(x, to: Shadow.self))
} 

This is quite a bit of fiddly machinery for what's ultimately a simple task: call objc_msgSend with a particular selector and calling convention. It gets even more hairy when we want to call objc_msgSend_super instead.

Could we have a direct way to express this?