Minimize the generic class metadata template by removing the
class header and base-class members. Add back the set of
information that's really required for instantiation.
Teach swift_allocateGenericClass how to allocate classes without
superclass metadata. Reorder generic initialization to establish
a stronger phase-ordering between allocation (the part that doesn't
really care about the generic arguments) and initialization (the
part that really does care about the generic arguments and therefore
might need to be delayed to handle metadata cycles).
A similar thing needs to happen for resilient class relocation.
Swift class metadata has a bit to distinguish it from non-Swift Objective-C
classes. The stable ABI will use a different bit so that stable Swift and
pre-stable Swift can be distinguished from each other.
No bits are actually changed yet. Enabling the new bit needs to wait for
other coordination such as libobjc.
rdar://35767811
