Use the `%target-swift-5.X-abi-triple` substitutions to compile the tests for
deployment to the minimum OS versions required for the APIs used in the tests,
instead of disabling availability checking.
Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of _Concurrency APIs,
instead of disabling availability checking.
The isolation checker was assuming that one can only be isolated to a
specific var, but that's not true for distributed actors -- because the
default parameter emitted by #isolation is a method call -- converting
the self into an any Actor.
We must handle this in isolation checker in order to avoid thinking
we're crossing isolation boundaries and making methods implicitly async
etc, when we're actually not.
resolves rdar://131874709
