Store referenced interface blocks in a cleaner data structure The previous code was hard to read since the referenced interface blocks stored a different type of node depending on if the interface block was instanced or not. BUG=angleproject:2267 TEST=angle_unittests Change-Id: Ie8fdb61a17280ca0875159702f819b884d08706b Reviewed-on: https://chromium-review.googlesource.com/839443 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>