[Concurrency] Fix task switch performance issue.

When using the new custom default executors, sometimes we end up
taking a long time to do a task switch.  This is happening because
the path the new code takes sometimes results in a concrete pointer
to the default global executor being in the executor tracking
information in `swift_task_switch()`, and if we try to switch to
a `nil` task executor (which _also_ means the default global executor),
we aren’t spotting that and we’re taking the slow path.

Essentially, we want to take the fast path in cases where we switch
from `nil` to the concrete default global executor and vice-versa.

rdar://156701386

Author: al45tair

stdlib/public/Concurrency/Actor.cpp

@@ -2391,13 +2391,15 @@ static bool mustSwitchToRun(SerialExecutorRef currentSerialExecutor,
   }
 
   // else, we may have to switch if the preferred task executor is different
-  auto differentTaskExecutor =
-      currentTaskExecutor.getIdentity() != newTaskExecutor.getIdentity();
-  if (differentTaskExecutor) {
-    return true;
-  }
+  if (currentTaskExecutor.getIdentity() == newTaskExecutor.getIdentity())
+    return false;
 
-  return false;
+  if (currentTaskExecutor.isUndefined())
+    currentTaskExecutor = swift_getDefaultExecutor();
+  if (newTaskExecutor.isUndefined())
+    newTaskExecutor = swift_getDefaultExecutor();
+
+  return currentTaskExecutor.getIdentity() != newTaskExecutor.getIdentity();
 }
 
 /// Given that we've assumed control of an executor on this thread,

stdlib/public/Concurrency/ExecutorBridge.h

@@ -23,6 +23,9 @@ namespace swift {
 extern "C" SWIFT_CC(swift)
 SerialExecutorRef swift_getMainExecutor();
 
+extern "C" SWIFT_CC(swift)
+TaskExecutorRef swift_getDefaultExecutor();
+
 #if !SWIFT_CONCURRENCY_EMBEDDED
 extern "C" SWIFT_CC(swift)
 void *swift_createDispatchEventC(void (*handler)(void *), void *context);

stdlib/public/Concurrency/ExecutorBridge.swift

@@ -106,6 +106,12 @@ internal func _getMainExecutorAsSerialExecutor() -> (any SerialExecutor)?
 #endif // SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
 #endif // os(WASI) || !$Embedded
 
+@available(StdlibDeploymentTarget 6.2, *)
+@_silgen_name("swift_getDefaultExecutor")
+internal func _getDefaultExecutorAsTaskExecutor() -> (any TaskExecutor)? {
+  return Task.defaultExecutor
+}
+
 @available(StdlibDeploymentTarget 6.2, *)
 @_silgen_name("swift_dispatchMain")
 internal func _dispatchMain()