use folly::void_t instead of std::void_t

Summary: folly/Traits.h defines a `void_t` alias in the `folly::` namespace. Just use that instead of specifying the one in `std::`.

Reviewed By: ispeters

Differential Revision: D25483712

fbshipit-source-id: 6be0435dfec600690cc90bc15fd3d2af17826173

folly/experimental/coro/Traits.h

@@ -206,7 +206,7 @@ constexpr bool promiseHasAsyncFrame_v = false;
 template <typename Promise>
 constexpr bool promiseHasAsyncFrame_v<
     Promise,
-    std::void_t<decltype(std::declval<Promise&>().getAsyncFrame())>> = true;
+    void_t<decltype(std::declval<Promise&>().getAsyncFrame())>> = true;
 
 } // namespace detail