Use is_invocable_r in FunctionRef
Use is_invocable_r in FunctionRef

Summary: [Folly] Use `is_invocable_r` in `FunctionRef` v.s. reimplementing it.

Reviewed By: ot

Differential Revision: D7769745

fbshipit-source-id: 3cb7bf9908cc6e93c7d19ace9a91c69ba14cdfae

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