[Concurrency] Add include of unistd.h to TaskGroup.cpp (#63265)

* [Concurrency] Add include of unistd.h to TaskGroup.cpp

rdar://104758975

Missing import caused build failures:

```
stdlib/public/Concurrency/TaskGroup.cpp:572:11: error: use of undeclared identifier 'STDERR_FILENO'
    write(STDERR_FILENO, message, strlen(message));
          ^
```

* Update TaskGroup.cpp

* Add missing _enqueue method in SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL mode

---------

Co-authored-by: Konrad `ktoso` Malawski <ktoso@apple.com>

Author: drexin

stdlib/public/Concurrency/TaskGroup.cpp

@@ -39,6 +39,11 @@
 #include <mutex>
 #endif
 
+#if defined(__APPLE__)
+#include <asl.h>
+#include <unistd.h>
+#endif
+
 #if SWIFT_STDLIB_HAS_ASL
 #include <asl.h>
 #elif defined(__ANDROID__)
@@ -1075,6 +1080,15 @@ static void _enqueueCompletedTask(NaiveTaskGroupQueue<ReadyQueueItem> *readyQueu
   readyQueue->enqueue(readyItem);
 }
 
+#if SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL
+static void _enqueueRawError(DiscardingTaskGroup *group,
+                             NaiveTaskGroupQueue<ReadyQueueItem> *readyQueue,
+                             SwiftError *error) {
+  auto readyItem = ReadyQueueItem::getRawError(group, error);
+  readyQueue->enqueue(readyItem);
+}
+#endif
+
 // TaskGroup is locked upon entry and exit
 void AccumulatingTaskGroup::enqueueCompletedTask(AsyncTask *completedTask, bool hadErrorResult) {
   // Retain the task while it is in the queue; it must remain alive until