Only refer to built-in function mangled names in lookup TFunction entries for built-ins don't need to store mangled names. BUG=angleproject:2267 TEST=angle_unittests Change-Id: Ia88e8bfa5357719c98d725dc3d00885dd59c9f59 Reviewed-on: https://chromium-review.googlesource.com/973241 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>