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
Hamish Knight
Pavel Yaskevich
Anthony Latsis