The lexical phase looks up symbols when it encounters an identifier.
Instead of duplicating this work when parsing non-constructor
functions, we now store the symbol looked up in the lexical phase in
TFunctionLookup.

This improves scores of the real world shader compiler perf test by
1-2%.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Idc99776571313d8b654910f9daaf9bf34a048228
Reviewed-on: https://chromium-review.googlesource.com/923725
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>