use forwarding reference for ReferencedSource range loop item

Summary:
When the elements in the containers are r-value references, or const l-value references, ReferencedSource fail to forward them.

This update generalize the interface to make it work for all type of references.

Reviewed By: ot, philippv

Differential Revision: D32807312

fbshipit-source-id: ec006aefd733e7f7fbf097a09b7cd50605ab0c9d

folly/gen/Base-inl.h

@@ -135,14 +135,14 @@ class ReferencedSource
 
   template <class Body>
   void foreach(Body&& body) const {
-    for (auto& value : *container_) {
+    for (auto&& value : *container_) {
       body(std::forward<Value>(value));
     }
   }
 
   template <class Handler>
   bool apply(Handler&& handler) const {
-    for (auto& value : *container_) {
+    for (auto&& value : *container_) {
       if (!handler(std::forward<Value>(value))) {
         return false;
       }