swift234

IRGen: Tighten up enum fixed-layout optimizations a bit

If an enum is public and @_fixed_layout, we can only use fixed-size
optimizations if the payload types are fixed-size in all resilience
domains.

However, if an enum is resilient or internal to a module, other
resilience domains cannot have knowledge of its layout, so we can
still use fixed-size optimizations if the payload types are known
to be fixed-size in the current resilience domain only.