Concurrency: Guard TG state with a mutex even with cooperative executor

`SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY` just means that the global
executor is cooperative, but it doesn't mean that the target platform is
always single-threaded. For example, on wasm32-unknown-wasip1-threads,
the global executor is cooperative, but users can still set up their own
TaskExecutor with multiple threads.

This patch guards the `TaskGroup` state with a mutex even with a
cooperative executor by respecting threading package instead. This
change effectively affects only wasm32-unknown-wasip1-threads.

Author: kateinoigakukun

stdlib/public/Concurrency/TaskGroup.cpp

@@ -292,7 +292,7 @@ class TaskGroupBase : public TaskGroupTaskStatusRecord {
   };
 
 protected:
-#if SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY || SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL
+#if SWIFT_THREADING_NONE || SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL
   // Synchronization is simple here. In a single threaded mode, all swift tasks
   // run on a single thread so no coordination is needed. In a task-to-thread
   // model, only the parent task which created the task group can