Fix std::move warning in folly/gen
Fix std::move warning in folly/gen

Summary:
The std::move prevents copy elision (NRVO), and gcc9 is warning about
it. Every compiler we use will do NRVO here, and in c++17 and later
the compiler is required to use the move constructor if it (for
whatever reason) fails to elide the copy.

Reviewed By: ot

Differential Revision: D19698783

fbshipit-source-id: 589556a0a88da0d84c607c9894399201dd7d6ae8

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly