This warning is produced if you annotate an enum with `@frozen` but
build without library evolution enabled. The problem is that if you
only build with library evolution enabled for release builds, this
leaves you with a warning for all debug builds. The downside of this
change is that if you never build with library evolution, you may have
this unnecessary annotation without noticing.
(currently spelled with an underscore to indicate its WIP state)
Later commits will handle imported enums correctly and implement the
checks for switch cases.
