This patch adds binding layout qualifier support for opaque types.
Binding layout qualifier on blocks is not yet supported.

This includes support for GLSL output and some minor simplification of
related functionality in ParseContext.

TEST=angle_unittests, dEQP-GLES31.functional.layout_binding.*
BUG=angleproject:1442

Change-Id: I53fb505b5a539bccee70613f3969fba81965ae84
Reviewed-on: https://chromium-review.googlesource.com/441586Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>