Just like `@preconcurrency` for concurrency, this attribute is going
to allow exhaustiveness error downgrades for enums that were retroactively
marked as `@extensible`.
This attribute controls whether cross-module access to the declaration
needs `@unknown default:` because it's allowed to gain new cases even
if the module is non-resilient.
If an enum comes from a different module that has `ExtensibleEnums`
feature enabled, unless it requires either `@unknown default:` or
`@frozen` because it is allowed to introduce new cases in the future
versions of the module.
