swift234

IRGen: Fix for fixed-layout enum with resilient payload

If an enum is fixed-layout in our resilience domain but not
universally fixed-layout, other resilience domains will use
runtime functions to project and inject payloads.

These expect to find the payload size in the metadata, so
emit it if the enum is not universally fixed-layout.

Note that we do know the payload size, so it is constant
for us; there's no runtime call required to initialize
the metadata.