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/973241Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>