This makes creating temporary symbols easy also outside of traversers.
This will be needed for improving variable initialization.

TEST=angle_unittests
BUG=chromium:735497

Change-Id: Id048fc338e0be6c76bb6f082421ae106618e5003
Reviewed-on: https://chromium-review.googlesource.com/707194
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>