When deriving `Hashable` and `Equatable` for enums, use
`Decl::isUnreachableAtRuntime()` to determine whether or not to insert
`_diagnoseUnavailableCodeReached()` traps for specific enum elements. This
fixes a bug where inappropriate traps were inserted for enum elements that are
unavailable for app extensions. It also fixes a bug where traps were inserted
when building a zippered library for macOS and enum elements were unavailable
on macOS but not for macCatalyst clients.
Resolves rdar://125371621
