gl_ViewportIndex is a GLSL built-in that's needed to implement
instanced multiview. It is a bit of a special case: it only exists in
desktop GLSL and not ESSL, and it shouldn't be exposed to the parser.
We add a new level to the symbol table that's hidden from the parser
to make adding this kind of builtins in AST transforms consistent with
the way ESSL builtins are supported.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I51b2d983950b38c8e85e4b6ed00c6b39f9b3cb03
Reviewed-on: https://chromium-review.googlesource.com/580953
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>