Don't add the same node pointer to the AST more than once, and assign
the right symbol ids to symbol nodes.

BUG=angleproject:1490
TEST=angle_unittests, angle_end2end_tests

Change-Id: I3f00e9234245fe4b81a2388df3f83e13c4c24856
Reviewed-on: https://chromium-review.googlesource.com/559534Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>