[Concurrency] Add missing 5.9 addTasks for DiscardingTaskGroups to gyb

Author: ktoso

stdlib/public/Concurrency/TaskGroup+addTask.swift.gyb

@@ -100,13 +100,35 @@ import Swift
 %      # -----------------------------------------------------------------------
 %      # === Baseline
 %      (
-%        '!SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY',
+%        '', # no #if
 %        [
 %          'TaskGroup',
 %          'ThrowingTaskGroup',
 %        ],
 %        [
 %          '@available(SwiftStdlib 5.1, *)',
+%          '#if SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY',
+%          '@available(*, unavailable, message: "Unavailable in task-to-thread concurrency model", renamed: "addTask(operation:)")',
+%          '#endif',
+%        ],
+%        ['addTask', 'addTaskUnlessCancelled'],
+%        [
+%          'priority: TaskPriority? = nil',
+%          # throws and ChildTaskResult will be adjusted per task group type
+%          'operation: sending @escaping @isolated(any) () async throws -> ChildTaskResult'
+%        ],
+%      ),
+%      (
+%        '', # no #if
+%        [
+%          'DiscardingTaskGroup',
+%          'ThrowingDiscardingTaskGroup',
+%        ],
+%        [
+%          '@available(SwiftStdlib 5.9, *)',
+%          '#if SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY',
+%          '@available(*, unavailable, message: "Unavailable in task-to-thread concurrency model", renamed: "addTask(operation:)")',
+%          '#endif',
 %        ],
 %        ['addTask', 'addTaskUnlessCancelled'],
 %        [