[Sema] Remove @unknown default requirement for package enums.
`@unknown default` in switch statements are required for resilient enums since they
might be modified with new fields in the future and modules defining the enums are
generally not built together with the consuming modules.
However, if the modules are in the same package, they are required to be built together,
thus the requirement for `@unknown default` can be skipped. This PR removes the need for
that, enabling less boilerplate. Note this change only impacts typecheck and not SIL gen.
Resolves rdar://130015149.
