Allow SDK overlays to add 'required' initializers.
Allow SDK overlays to add 'required' initializers.

Removes the initWithBool: hack from the Clang importer. We can now
express NSNumber's conformance to the BooleanLiteralConvertible
protocol in the overlay.

Swift SVN r21976

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation