swift234

Make metatype layout compatible with struct objc_class.

The principal difficulty here is that we need accessing the
value witness table for a type to be an efficient operation,
but there (obviously) isn't a VWT field for ObjC classes.
Placing this field after the metatype would tend to bloat
metatypes by quite a bit.  Placing it before is best, but
it introduces an unfortunate difference between the address
point of a metatype and the address of the global symbol.
That, however, can be fixed with appropriate linker support.
Still, for now this is rather unfortunately over-subtle.

Swift SVN r3307