[parser] refactor JSX child/closing parsing
[parser] refactor JSX child/closing parsing

Summary: inlining `element_or_closing` into its only caller is simpler since the element and closing cases are actually pretty different and the intermediate `element_or_closing` type just got in the way.

Reviewed By: gabelevi

Differential Revision: D16542090

fbshipit-source-id: 46c8003a8c956264c2a5aae9b4cdc7d433894966

samwgoldman/flow,facebook/flow,facebook/flow,gabelevi/flow,mroch/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,mroch/flow,mroch/flow,mroch/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,mroch/flow,facebook/flow,facebook/flow