Remove unreferenced struct types from the AST This expands pruning unreferenced variables so that unreferenced named struct types can also be removed from the AST. Includes a small cleanup in GLSL output so that the output code matching tests can test against clean output. BUG=chromium:786535 TEST=angle_unittests Change-Id: I20974ac99a797e478d82f9203c179d2d58fac268 Reviewed-on: https://chromium-review.googlesource.com/779519 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>