Unify 'MaybeThrow' parsing logic
Unify 'MaybeThrow' parsing logic

Summary: Throws is always optional, but it had 3 different implementations of that in the parser. Now there is only one.

Reviewed By: yfeldblum

Differential Revision: D28880862

fbshipit-source-id: 31d5bc6593a97736dad8224bd2f9160be62374a1

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