Refactor function call node creation 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/433217 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>