Issue Description:

The following piece of code compiles just fine on Darwin:

import Foundation

class MyKeyedUnarchiver: NSKeyedUnarchiver {
    override init(forReadingWith data: Data) {
        super.init(forReadingWith: data)
    }
}

_ = MyKeyedUnarchiver(forReadingWith: Data())

But fails on Linux (4.1, ubuntu 14.04) with:

Compile Swift Module 'KeyedUnarchiverSubclassing' (1 sources)
/Users/gmilos/Repos/KeyedUnarchiverSubclassing/Sources/KeyedUnarchiverSubclassing/main.swift:4:14: error: initializer does not override a designated initializer from its superclass
override init(forReadingWith data: Data) {
~~~~~~~~ ^
Foundation.NSKeyedUnarchiver:6:24: note: attempt to override convenience initializer here
public convenience init(forReadingWith data: Foundation.Data)
^
/Users/gmilos/Repos/KeyedUnarchiverSubclassing/Sources/KeyedUnarchiverSubclassing/main.swift:5:9: error: must call a designated initializer of the superclass 'NSKeyedUnarchiver'
super.init(forReadingWith: data)
^
Foundation.NSKeyedUnarchiver:6:24: note: convenience initializer is declared here
public convenience init(forReadingWith data: Foundation.Data)