The patch decouples the list of unmangled built-ins' names into levels
based on the ESSL version they are from. This is required
because ESSL31 and above built-ins' names should not conflict with
user-declared functions in ESSL3 shaders.

BUG=angleproject:1666
TEST=angle_unittest

Change-Id: I731918a058629c7b6d9d15eb7eac19ce47280315
Reviewed-on: https://chromium-review.googlesource.com/420324Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>