Resubmitting now that separate fix for updating mangled names of types
has been merged. A unit test is added to make sure that the same
regression doesn't happen again.

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: Ie9b6aaee8185b945c03657b13d9513cc55cd2a9f
Reviewed-on: https://chromium-review.googlesource.com/303601Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>