Always create TFunctions for function call nodes This simplifies code and ensures that nodes get consistent data. In the future function call nodes could have a pointer to the TFunction instead of converting the same information into a different data structure. BUG=angleproject:2267 TEST=angle_unittests, angle_end2end_tests Change-Id: Ic0c24bb86b44b9bcc4a5da7f6b03701081a3af5c Reviewed-on: https://chromium-review.googlesource.com/824606 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>