Clean up some TODOs in the code as well as add tests to make sure that
ANGLE treats expressions indexing into constant arrays as constant
expressions in different contexts. This complements the existing tests
in ConstantFoldingTest.

BUG=angleproject:2298
TEST=angle_unittests

Change-Id: I779bf03891f7d06f14d293e69101c05d7dbf57b6
Reviewed-on: https://chromium-review.googlesource.com/1254067Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>