[Concurrency] Fix CooperativeExecutor to not loop forever.

If a job enqueues another job on the executor, we might never leave
the inner `while` loop in the `run()` method.  Fix this by taking
the contents of the run queue and only running those jobs in the
queue at the time we enter the inner loop.

Author: al45tair

stdlib/public/Concurrency/CooperativeExecutor.swift

@@ -248,7 +248,8 @@ extension CooperativeExecutor: RunLoopExecutor {
       #endif
 
       // Now run any queued jobs
-      while let job = runQueue.pop() {
+      var runQ = runQueue.take()
+      while let job = runQ.pop() {
         unsafe ExecutorJob(job).runSynchronously(
           on: self.asUnownedSerialExecutor()
         )

stdlib/public/Concurrency/PriorityQueue.swift

@@ -40,6 +40,15 @@ struct PriorityQueue<T> {
     self.compare = compare
   }
 
+  /// Take the queue.
+  ///
+  /// This returns a copy of the queue, and empties the original.
+  mutating func take() -> PriorityQueue<T> {
+    var q = PriorityQueue(compare: self.compare)
+    swap(&self, &q)
+    return q
+  }
+
   /// Push an item onto the queue.
   ///
   /// Parameters: