[Concurrency] repair cooperative global executor (#39092)

`SWIFT_STDLIB_SINGLE_THREADED_RUNTIME` mode has been broken for a long time.

This patch guards some includes and use of libdispatch headers so that platforms
that doesn't support libdispatch can build cooperative executor runtime.
And fixed missing implementations for cooperative mode.

stdlib/public/Concurrency/GlobalExecutor.cpp

@@ -59,12 +59,15 @@
 #include "TaskPrivate.h"
 #include "Error.h"
 
+#if !SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
 #include <dispatch/dispatch.h>
 
 #if !defined(_WIN32)
 #include <dlfcn.h>
 #endif
 
+#endif
+
 using namespace swift;
 
 SWIFT_CC(swift)
@@ -180,7 +183,7 @@ void swift::donateThreadToGlobalExecutorUntil(bool (*condition)(void *),
   while (!condition(conditionContext)) {
     auto job = claimNextFromJobQueue();
     if (!job) return;
-    job->run(ExecutorRef::generic());
+    swift_job_run(job, ExecutorRef::generic());
   }
 }
 
@@ -405,21 +408,35 @@ void swift::swift_task_enqueueMainExecutor(Job *job) {
     swift_task_enqueueMainExecutorImpl(job);
 }
 
+#if !SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
 void swift::swift_task_enqueueOnDispatchQueue(Job *job,
                                               HeapObject *_queue) {
   JobPriority priority = job->getPriority();
   auto queue = reinterpret_cast<dispatch_queue_t>(_queue);
   dispatchEnqueue(queue, job, (dispatch_qos_class_t)priority, queue);
 }
+#endif
+
+#if SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
+static HeapObject _swift_mainExecutorIdentity;
+#endif
 
 ExecutorRef swift::swift_task_getMainExecutor() {
+#if SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
+  return ExecutorRef::forOrdinary(&_swift_mainExecutorIdentity, nullptr);
+#else
   return ExecutorRef::forOrdinary(
            reinterpret_cast<HeapObject*>(&_dispatch_main_q),
            _swift_task_getDispatchQueueSerialExecutorWitnessTable());
+#endif
 }
 
 bool ExecutorRef::isMainExecutor() const {
+#if SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
+  return Identity == &_swift_mainExecutorIdentity;
+#else
   return Identity == reinterpret_cast<HeapObject*>(&_dispatch_main_q);
+#endif
 }
 
 #define OVERRIDE_GLOBAL_EXECUTOR COMPATIBILITY_OVERRIDE