[Concurrency] repair cooperative global executor

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

# Conflicts:
#	stdlib/public/Concurrency/GlobalExecutor.cpp

Author: kateinoigakukun

stdlib/public/Concurrency/GlobalExecutor.cpp

@@ -447,9 +447,13 @@ void swift::swift_task_enqueueOnDispatchQueue(Job *job,
 }
 #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::generic();
+  return ExecutorRef::forOrdinary(&_swift_mainExecutorIdentity, nullptr);
 #else
   return ExecutorRef::forOrdinary(
            reinterpret_cast<HeapObject*>(&_dispatch_main_q),
@@ -459,7 +463,7 @@ ExecutorRef swift::swift_task_getMainExecutor() {
 
 bool ExecutorRef::isMainExecutor() const {
 #if SWIFT_CONCURRENCY_COOPERATIVE_GLOBAL_EXECUTOR
-  return isGeneric();
+  return Identity == &_swift_mainExecutorIdentity;
 #else
   return Identity == reinterpret_cast<HeapObject*>(&_dispatch_main_q);
 #endif