Replaces generic `expression is 'async' but is not marked with 'await`
diagnostic with a tailed one for cases where there is an access to an
actor-isolated value outside of its actor without `await` keyword.
This makes the diagnostics for async and sync contexts consistent
and actually identifies a problem instead of simply pointing out
the solution.
Resolves: rdar://151720646
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.
