Most of the functions were just simple wrappers around node
constructors. Dropping this extra redirection makes the code simpler.

The fold() functions of node types are simplified, so that if the node
can't be folded the pointer to the node itself is returned. This makes
the code in ParseContext more straightforward.

The few remaining functions in Intermediate are a bit more complex so
they should be handled separately, but they'll be removed eventually
as well.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I85e11919d1f62358cfba9c011b841e32bc25402f
Reviewed-on: https://chromium-review.googlesource.com/563393Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>