Direct initialize semaphoreWaiter (#1554)

Summary:
Before this patch, it relies on the c++17 guaranteed copy elision, since `SemaphoreBase::Waiter` is NOT copy constructible due to `std::atomic<intptr_t>`.

This patch makes it compile even using `-std=c++14`.

Pull Request resolved: https://github.com/facebook/folly/pull/1554

Differential Revision: D27505938

Pulled By: yfeldblum

fbshipit-source-id: 6c26f82f77a24236a1233cff7921cd6b9f53bd09

folly/fibers/SemaphoreBase.cpp

@@ -167,8 +167,7 @@ namespace {
 
 class FutureWaiter final : public fibers::Baton::Waiter {
  public:
-  explicit FutureWaiter(int64_t tokens)
-      : semaphoreWaiter(SemaphoreBase::Waiter(tokens)) {
+  explicit FutureWaiter(int64_t tokens) : semaphoreWaiter(tokens) {
     semaphoreWaiter.baton.setWaiter(*this);
   }