[ast-builder] Support building methods with `Ast_builder`
[ast-builder] Support building methods with `Ast_builder`

Summary: Added this helper builder function to make creating method definition less verbose.

Reviewed By: gkz

Differential Revision: D29345392

fbshipit-source-id: 90f2e5f2f5066c26349db9b4eeccf29b1f7d4cf7

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