[Concurrency] Fix races/overflows in TaskGroup implementation.

statusCompletePendingReadyWaiting(), offer(), and poll() did a one-off compare_exchange_strong which could fail if the group was concurrently cancelled. Put these into loops so that they are retried when needed.

DiscardingTaskGroup creation passed the group result type as the task result type. waitAll() would then use the group result type when collecting task results. Since the task result type is always Void in this case, this would overflow the result buffer if the group result type was larger. This often works as it writes into the free space of the task allocator, but can crash if it happens to be at the end of a page or the group result type is particularly large.

rdar://151663730

Author: mikeash

stdlib/public/Concurrency/DiscardingTaskGroup.swift

@@ -80,7 +80,7 @@ public func withDiscardingTaskGroup<GroupResult>(
     discardResults: true
   )
 
-  let _group = Builtin.createTaskGroupWithFlags(flags, GroupResult.self)
+  let _group = Builtin.createTaskGroupWithFlags(flags, Void.self)
   var group = DiscardingTaskGroup(group: _group)
   defer { Builtin.destroyTaskGroup(_group) }
 
@@ -108,7 +108,7 @@ public func _unsafeInheritExecutor_withDiscardingTaskGroup<GroupResult>(
     discardResults: true
   )
 
-  let _group = Builtin.createTaskGroupWithFlags(flags, GroupResult.self)
+  let _group = Builtin.createTaskGroupWithFlags(flags, Void.self)
   var group = DiscardingTaskGroup(group: _group)
   defer { Builtin.destroyTaskGroup(_group) }
 
@@ -347,7 +347,7 @@ public func withThrowingDiscardingTaskGroup<GroupResult>(
       discardResults: true
   )
 
-  let _group = Builtin.createTaskGroupWithFlags(flags, GroupResult.self)
+  let _group = Builtin.createTaskGroupWithFlags(flags, Void.self)
   var group = ThrowingDiscardingTaskGroup<Error>(group: _group)
   defer { Builtin.destroyTaskGroup(_group) }
 
@@ -378,7 +378,7 @@ public func _unsafeInheritExecutor_withThrowingDiscardingTaskGroup<GroupResult>(
       discardResults: true
   )
 
-  let _group = Builtin.createTaskGroupWithFlags(flags, GroupResult.self)
+  let _group = Builtin.createTaskGroupWithFlags(flags, Void.self)
   var group = ThrowingDiscardingTaskGroup<Error>(group: _group)
   defer { Builtin.destroyTaskGroup(_group) }