Keep track of shader built-in declarations separately per ESSL version 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/420324 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>