Instead, 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.
the context do not exit to nonisolated.
Setting the `mayExitToNonisolated` during isolated parameter checking was
done after a check that bails out early when the isolated argument matches
the isolated paramter of the caller, so the actor isolation checker was
accidentally considering the call as exiting to a nonisolated context. This
lead to bogus diagnostics during the region isolation pass.
