swift234

Change the layout of the extended class-object header: add
a flags field, add an instance address point field, and reserve
some additional space.

This change must be coordinated with a corresponding change
to ObjC runtime bits in libarclite; without this, dynamic
subclassing features like KVO will break.

The actual contents of the new fields can change without
bothering the ObjC runtime.

Swift SVN r20183