let cpp2 access_field be const-invocable
let cpp2 access_field be const-invocable

Summary: Like any good invoker.

Reviewed By: Mizuchi

Differential Revision: D34815305

fbshipit-source-id: dc5b3ad89940bdced21e70cf2fcf1da21039fffd

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift