TIntermNode classes now contain all the logic for setting node types.

Changing the constant values of constant union nodes from outside is
also not necessary anymore.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ic10d41b1e5f93152df440a655057591dc1b783b0
Reviewed-on: https://chromium-review.googlesource.com/863626Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>