Frontend: Enable `$GlobalActors` feature by default.
Global actors have been an accepted language feature for a couple of compiler releases now, but the feature definition was associated with the `--enable-experimental-concurrency` flag. This caused some `.swiftinterface`s containing global actor declarations to be unparsable because the logic for surrounding declarations with the `$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
