folly: accept argument to `try_emplace_with` with moveable value

Summary: This allows `try_emplace_with` to receive objects by copy-ellision or specific moves; without it, any attempts to pass an object would result in a copy.

Reviewed By: praihan, luciang

Differential Revision: D26008949

fbshipit-source-id: b43d2dc941c7415e2189668a2b577cc0f1165665

folly/synchronization/DelayedInit.h

@@ -80,7 +80,7 @@ struct DelayedInit {
    * then the provided function is not called.
    */
   template <typename Func>
-  T& try_emplace_with(Func func) const {
+  T& try_emplace_with(Func&& func) const {
     call_once(storage_.init, [&]() mutable {
       new (std::addressof(storage_.value)) T(func());
     });