The compiler synthesizes an `unownedExecutor` property for every `actor` type.
It was also synthesizing availability attributes for this property by
intersecting the availability of the actor type and the `UnownedExecutor` type.
This may have been necessary early in the development of Swift Concurrency but
now it amounts to little more than a convoluted procedure that must result in
repeating the availability of the actor on one of its properties (the property
can never be less available than the actor itself).
Allan Shortlidge