Issue Description:

Swift has no official way to mark an API as Obj-C–only such that it cannot be called from Swift. An idiom adopted by my team is to mark the API as obsoleted since Swift 1.0, thus preventing it from being called from Swift, but still allowing it to be called from Obj-C. The problem is an API marked obsoleted in this fashion cannot call another API marked obsoleted, which is rather restrictive. Here's an example:

import Foundation
class Foo: NSObject {
    @available(swift, obsoleted: 1.0)
    @objc func apiOnlyForObjC() -> ClassOnlyForObjC {
        return ClassOnlyForObjC()
    }
    @objc(FooResult)
    @available(swift, obsoleted: 1.0)
    class ClassOnlyForObjC: NSObject {}
}

Another example would be when I'm providing several variants of an @objc method with different numbers of parameters to simulate optional parameters, I can't have the simpler versions call the more complex versions because they're all marked as obsoleted.