Now that unique ids of all builtins are compile-time constants, we can
use them to look up functions in BuiltInFunctionEmulator. This is
simpler than using a custom struct with the name and parameters for
identifying functions.

This requires that we store a reference to a TFunction in those
TIntermUnary nodes that were created based on a function.

This decreases shader_translator binary size by about 6 KB on Windows.

BUG=angleproject:2267
BUG=chromium:823856
TEST=angle_unittests

Change-Id: Idd5a00c772c6f26dd36fdbbfbe161d22ab27c2fe
Reviewed-on: https://chromium-review.googlesource.com/995372Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>