add builder for immutable interface

Summary: add builder for immutable interface. Otherwise I need either to use bean like setter/getter. @lif didn't like that....

Test Plan: generated interface: https://phabricator.fb.com/D1165799

Reviewers: andrewcox

CC: lif

Differential Revision: https://phabricator.fb.com/D1178006

Task ID: 3638666