This cannot be supported for the same reasons struct and class stored
properties with potential unavailability cannot be supported.
It is possible that some users relied on this, because it worked in
some cases where the type metadata for the potentially unavailable
type was not actually needed for anything (eg, if the type was @frozen
and trivial, or a simple class reference). If this becomes an issue
we can downgrade this to a warning.
Note that just like the previous fix for stored properties, we do
still allow writing an '@available' attribute on the enum element if
the OS version is not any newer than the deployment target. This allows
annotating APIs with the OS version where they were introduced.
