Apply LWG 2021.  This is only a documentation change.  Also bringing c++1y status page up to date.

llvm-svn: 191141
diff --git a/libcxx/include/future b/libcxx/include/future
index 9c4c70d..334eaeb 100644
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -309,11 +309,11 @@
 };
 
 template <class F, class... Args>
-  future<typename result_of<F(Args...)>::type>
+  future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   async(F&& f, Args&&... args);
 
 template <class F, class... Args>
-  future<typename result_of<F(Args...)>::type>
+  future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   async(launch policy, F&& f, Args&&... args);
 
 template <class> class packaged_task; // undefined