Desktop GLSL builtins can be accessed through functions in
BuiltIn_autogen.h. They don't need to be included in symbol table
lookups.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I8ba188a0d2584353e34159c2732c9e2bd420c168
Reviewed-on: https://chromium-review.googlesource.com/964447Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>