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>