Don't copy function name strings unnecessarily and traverse function
body nodes manually to avoid some extra traversal steps.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ie010aabcb8bc78fa6abce397ea2bdd9092e74187
Reviewed-on: https://chromium-review.googlesource.com/847552Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>