It should not be possible to use globals as l-values in global initializers.

This change was tested extensively with popular WebGL content, with no
regressions found.

TEST=angle_unittests
BUG=angleproject:988

Change-Id: I21ab731eb1d92aeae25795856ccae280792ad1f0
Reviewed-on: https://chromium-review.googlesource.com/283910Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>