Index symbols by id in OutputHLSL This is cleaner than indexing them by their name string. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I0d0ef5e3f6a3f26c94f096b086cdf3da40d495e4 Reviewed-on: https://chromium-review.googlesource.com/845559 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>