In modern ObjC Protocol's object-ness is vestigial, and its class object isn't even visible from newer runtimes, so we can't use it as type metadata. Import it as a foreign class so that we make our own artificial metadata. Fixes <rdar://problem/17303759>.
Swift SVN r18882
<rdar://problem/17303759> -- we need to do something special for the Protocol type's metadata, because its ObjC class object is hidden.
Swift SVN r18871