This makes function call node creation code simpler and more type
safe. It also prepares for further simplification by removing usage of
TFunction in places where the arguments node is sufficient.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I75d9e059bb32c475487f0be24e40ac0d78012d86
Reviewed-on: https://chromium-review.googlesource.com/433217Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>