Previously, constness of indexing expressions did not take the index
expression into account. Now constness correctly takes into account both
the base expression and the index expression.

Setting the type of expressions that index arrays is also simplified.

BUG=angleproject:1170
TEST=angle_unittests

Change-Id: Ie2d122020cc252655ab0eea96886b9f85931b80a
Reviewed-on: https://chromium-review.googlesource.com/303350Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>