Issue Description:

I wonder whether this sort of thing could be made to compile?

let sc : NSSecureCoding =  "howdy"
let c : NSCoding = 1

The compiler could reason: Well, this protocol is an Objective-C class protocol, so it is analogous to a type AnyObject. If this String/Int were assigned where an AnyObject is expected, I would cross the bridge to NSString / NSNumber. If I do that, do I get an NSSecureCoding / NSCoding adopter? Yes!

As it is, I have to say `as NSString`, `as NSNumber`, which is surprising considering how we usually cross the bridge from Swift implicitly without a murmur.

The issue arises in real life particularly because a number of Cocoa APIs do specify an NSSecureCoding parameter, but the programmer typically does not know that, passes a String or an Int, and is surprised when that doesn't work.