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.
This reverts commit 4323d2fa26, reversing
changes made to 451b902cd5.
This caused linking errors on the swift source compat suite in the
Sourcery project.
rdar://86256970
For `@objc async` methods, all of the important information about the
"result" type and whether it throws or not is within the type of the
completion handler block. Enable extended type information for the
generated Objective-C metadata of such methods, so that clients can
understand how to invoke them using reflection facilities.
Implements rdar://73048313.
