It could be confusing to adopters who were led to believe by the types
that they should "just" implement the sharedUnownedExecutor property,
but insead they have to implement the unownedExecutor on the specific
actor type.
Adding documentation clarify this as well as a simple test that
exercises this explicitly; We seem to have much coverage of main actor,
but not so much of custom executor global actors.
The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
Based on the discussion in the first review of the global actors
proposal, introduce a `GlobalActor` protocol that describes types that
can be global actors. Introduce this protocol, make `@globalActor`
types implicitly conform to it, and remove all of the bespoke
validation logic that was used to check the "shared" member.
Addresses rdar://79339591
