Issue Description:

The following crashes with the error "Could not cast value of type 'SwiftObjCProtocolBug.Test' (0x1002c6988) to 'SwiftObjCProtocolBug.Foo' (0x1005fd040)."

import Foundation

@objc protocol Foo { }
protocol Bar: Foo { }
class Test: Bar { }

func magic<T: Foo>(x: T) -> T {
    let y = x as Any
    return y as! T
}

print(magic(Test() as Foo))

Workaround: Add a redundant conformance to `Foo` when `Test` is declared. Oddly, the compiler doesn't warn us about this. Note that the redundant conformance cannot be added in an extension due to SR-2078.