Discarding task groups aren't available in the 5.8-aligned OS's,
resulting in a loader error at launch if you try to use them.
Bumping the availability attributes to 5.9 on everything in
DiscardingTaskGroup.swift
* [Concurrency] Fix too optimistic bail-out-when-empty, discarding group
may need to emit an error out of such waitAll attempt, if a previous
error was already stored.
* [Concurrency] DiscardingTG error thrown from body always wins
* Offering body error must be done while holding lock
* Must not modify waitingTask context outside lock
* wip on trying to fix by changing the offer
* fix bug in resuming tasks in offer/resume task in discarding group
* Fix#63147 waitForAll impl in ThrowingTaskGroup, used to not wait enough
rdar://104507347
* done
* debug
* explicitly declare virtual destructors
* detach child task after filling result; dont mutate context outside of lock
* Add pending count overflow protection to TaskGroup
* unlock async_taskgroup_void_neverConsumingTasks again rdar://104332560
* missing include on windows for error reporting
* fix ARM_ARCH_7K workaround snippet
