swift234

IRGen: Some field offset globals can be constant

It is questionable if emitting globals for constant offsets is of
any use; the only scenario where one could not compute the offset
directly in IRGen is if the class has a fixed-size layout inside
its own resilience domain, but has a non-fixed layout outside,
*and* we're doing direct stored property access.

Right now we don't plan on doing this, but it might come up in
the future with more exotic resilience domain configurations or
perhaps optimizations.