This change will make it easier to split types of TIntermAggregate
nodes representing functions and function calls into different node
classes.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I730aa7858fe31fda86218fc685980c6ad486f5e0
Reviewed-on: https://chromium-review.googlesource.com/394706Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>