Function prototypes now have their own class TIntermFunctionPrototype.
It's only used for prototypes, not function parameter lists.

TIntermAggregate is still used for parameter lists and function calls.

BUGS=angleproject:1490
TEST=angle_unittests

Change-Id: I6e246ad00a29c2335bd2ab7f61cf73fe463b74bb
Reviewed-on: https://chromium-review.googlesource.com/427944Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>