[SE-0300] Add Continuation.resume(), no argument, for Void return type
[SE-0300] Add Continuation.resume(), no argument, for Void return type

This change adds `*Continuation<Void>.resume()` to the Continuation proposal. This is useful when a continuation needs to return `Void` because otherwise you have to say `cc.resume(returning: ())`, and it's a bit odd to pass an instance of `Void` to a function.

apple/swift-evolution,natecook1000/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,abertelrud/swift-evolution,abertelrud/swift-evolution