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.
When build-script is given `--back-deploy-concurrency`, also use that
to build other parts of Swift with the back-deployed versions:
* The compiler allows async and actors to be defined with the
back-deployed availability, e.g., the same as `-Xfrontend
-enable-experimental-back-deploy-concurrency`. (The latter will go
away soon)
* The standard library unit testing framework and distributed actors
library are build with the older OS versions.
* The tests use the older OS versions, with some adjustments to make
them agnostic to the back-deployment setting.
Allow an actor to be exposed to Objective-C via `@objc` without
inheriting from `NSObject`, and remove the loophole that allowed
actors to inherit from `NSObject`.
Fixes rdar://78333614
Treat actors as being semantically `final` throughout the type checker.
This allows, for example, a non-`required` initializer to satisfy a
protocol requirement.
We're leaving the ABI open for actor inheritance should we need it.
Addresses rdar://78269551.
The check that limited inference of actor isolation meant that we were
incorrectly computing actor isolation for (e.g.) overrides when
parsing from Swift interfaces. Only limit inference for cases where we
are guaranteed to synthesize an attribute by inference.
