Fix semaphore usage in UnboundedBlockingQueue

Summary:
[Folly] Fix semaphore usage in `UnboundedBlockingQueue`.

Ensure that `wait` is paired 1:1 with `post`.

Reviewed By: davidtgoldblatt, magedm

Differential Revision: D14537954

fbshipit-source-id: def54263da1a33c1b3ffa1de237a2b786572759a

folly/executors/task_queue/UnboundedBlockingQueue.h

@@ -33,21 +33,15 @@ class UnboundedBlockingQueue : public BlockingQueue<T> {
   }
 
   T take() override {
-    T item;
-    while (!queue_.try_dequeue(item)) {
-      sem_.wait();
-    }
-    return item;
+    sem_.wait();
+    return queue_.dequeue();
   }
 
   folly::Optional<T> try_take_for(std::chrono::milliseconds time) override {
-    T item;
-    while (!queue_.try_dequeue(item)) {
-      if (!sem_.try_wait_for(time)) {
-        return folly::none;
-      }
+    if (!sem_.try_wait_for(time)) {
+      return folly::none;
     }
-    return std::move(item);
+    return queue_.dequeue();
   }
 
   size_t size() override {