Fix propagate_const under gcc49
Fix propagate_const under gcc49

Summary: [Folly] Fix `propagate_const` under gcc49.

Reviewed By: andrewjcg

Differential Revision: D7230851

fbshipit-source-id: 0cd4df15d5a0f7ae06e26a82ca725b30d479f458

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