Unmangled names of built-ins are not inserted to the symbol table in
a normal way, so the function to do that is not needed.

BUG=angleproject:2267
TEST=compile ANGLE

Change-Id: I68bae9a1f762ecc69ab31f04458a38c941b9d74c
Reviewed-on: https://chromium-review.googlesource.com/922202Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>