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 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.
Rather than blanket-disabling concurrency tests when we aren't using a
just-built concurrency library, enable them whenever we have a
suitable concurrency runtime, either just-built, in the OS, or via the
back-deployment libraries.
The proper handling of task group child tasks is that:
- if it completes a waiting task immediately, we don't need to retain it
- we just move the value to the waiting task and can destroy the task
- if we need to store the ready task and wait for a waiting task (for a
task that hits `await group.next()`) then we need to retain the ready
task.
- as the waiting task arrives, we move the value from the ready task
to the waiting task, and swift_release the ready task -- it will now
be destroyed safely.
This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
The `try await` ordering is both easier to read and indicates the order
of operations better, because the suspension point occurs first and
then one can observe a thrown error.
