swift234

IRGen: Make case numbering consistent for enums with empty payloads

If an enum case has a payload but the unsubstituted payload type is
zero-sized, we would convert the case into a no-payload case.
This was valid when the only invariant that had to be preserved
is that an enum's layout is the same between all substitutions
of a generic type.

However this is now wrong if the payload type is resiliently-sized,
because other resilience domains may not have knowledge that it is
zero-sized.

The new utility methods will also be used in class layout.