[enums] Add enums to AST builder
[enums] Add enums to AST builder

Summary: Adds EnumDeclaration and associated nodes to the AST builder.

Reviewed By: gabelevi

Differential Revision: D15796981

fbshipit-source-id: 50f62599edc12cd5348d4d314921b6ee6a992677

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