This enables inserting helper functions as an AST transformation.

BUG=angleproject:1116
TEST=angle_unittests

Change-Id: I169d4d3a726d0e389cb3444fe9dfb4c6c5d80155
Reviewed-on: https://chromium-review.googlesource.com/290513Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>