In an expression like a[ind]++, a[ind] is required to be an l-value but
ind is not. Reset the l-value required flags before traversing the index
of an indexing operation, so that this is accurately tracked. After the
index has been traversed, the previous state of the l-value required
flags is restored.

New tests are added to angle_unittests cover this functionality.

TEST=angle_unittests
BUG=angleproject:1116

Change-Id: I8929ec01e85e672c83ef7d385e455b7df8682f4b
Reviewed-on: https://chromium-review.googlesource.com/290561Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>