Implement @unowned(unsafe).

This was part of the original weak design that
there was never any particular reason to rush the
implementation for.  It's convenient to do this now
so that we can use it to implement Unmanaged<T> for
importing CF types.

Swift SVN r16693

include/swift/Serialization/ModuleFormat.h

@@ -184,7 +184,8 @@ using AssociativityField = BCFixed<2>;
 enum Ownership : uint8_t {
   Strong = 0,
   Weak,
-  Unowned
+  Unowned,
+  Unmanaged,
 };
 using OwnershipField = BCFixed<2>;