[Concurrency] Fix too optimistic TaskGroup bail-out-when-empty, (#63016)

* [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

Author: ktoso

stdlib/public/Concurrency/DiscardingTaskGroup.swift

@@ -86,6 +86,7 @@ public func withDiscardingTaskGroup<GroupResult>(
   let result = await body(&group)
 
   try! await group.awaitAllRemainingTasks() // try!-safe, cannot throw since this is a non throwing group
+
   return result
   #else
   fatalError("Swift compiler is incompatible with this SDK version")