Merge pull request #63017 from ktoso/pick-wip-waitAll-race

[5.8][Concurrency] Fix too optimistic TaskGroup bail-out-when-empty

Author: DougGregor

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")