Issue Description:

The following code should print true twice, but the is Base check erroneously fails:

import Foundation

@objc protocol Base {}

protocol Some : Base {}

class Nada : NSObject, Some {}

let x: NSObject = Nada()

print("Nada is Some? \(x is Some)")
print("Nada is Base? \(x is Base)")

I suspect that, when we emit the ObjC metadata for Nada, we fail to walk the super-protocols of the protocols that Nada conforms to in order to find ObjC protocol conformances that need to be recorded.