add assert to verify const initializer type
add assert to verify const initializer type

Reviewed By: iahs

Differential Revision: D23811648

fbshipit-source-id: ddd97c5eab19007f34e814c35ed01bc7e411a308

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