Work around a bug in MSVC name lookup within templated friend contexts

Summary: MSVC has a bug that causes it to think that `once_flag` in this context resolves to the `using` statement further up, rather than `detail::once_flag`.

Reviewed By: yfeldblum

Differential Revision: D6376501

fbshipit-source-id: 481dbd75ed21f3d519bd920258fa743f314668ad

folly/synchronization/CallOnce.h

@@ -83,12 +83,12 @@ class once_flag {
 
   template <typename Mutex_, typename Callable, class... Args>
   friend void ::folly::call_once(
-      once_flag<Mutex_>& flag,
+      detail::once_flag<Mutex_>& flag,
       Callable&& f,
       Args&&... args);
   template <typename Mutex_, typename Callable, class... Args>
   friend void call_once_impl_no_inline(
-      once_flag<Mutex_>& flag,
+      detail::once_flag<Mutex_>& flag,
       Callable&& f,
       Args&&... args);