Don't emit placeholders for field offsets and vtable entries,
since they were always null. Instead, calculate the final size
of class metadata at runtime using the size of the superclass
metadata and the number of immediate members, and only copy
this prefix from the template to the instantiated metadata,
zero-filling the rest.
For this to work with non-generic resilient classes and
non-generic subclasses of generic classes, we need a new
runtime entry point to relocate non-generic class metadata,
calculating its size at runtime using the same strategy.
Once generic type metadata includes arguments from all outer contexts,
we need to know how many arguments there are at each nesting depth in
order to properly reconstruct the type name from metadata.
Instead of emitting the vtable statically, copy it from the
superclass, and fill in method overrides, if any.
For now this is only done for classes initialized by
swift_initClassMetadata_UniversalStrategy(); I need to
add a new entry point for classes with static field
layout but a dynamic vtable.
