Global actors have been an accepted language feature for a couple of compiler releases now, but the feature definition was associated with the `--enable-experimental-concurrency` flag. This caused some `.swiftinterface`s containing global actor declarations to be unparsable because the logic for surrounding declarations with the `$GlobalActors` feature guard was incomplete (for example, classes with a global actor attribute were not guarded even though the declarations of global actor types were).
Rather than trying to fix the logic of `usesFeatureGlobalActors()`, enable it by default.
Adds a test that demonstrates that modules defining and using public global actors produce module interfaces that can be parsed successfully.
Resolves rdar://100150703
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.
Becca Royal-Gordon
Allan Shortlidge
Karoy Lorentey
Doug Gregor
Alexis Laferrière
Mike Ash
David Zarzycki
Kavon Farvardin