Avoid redundant symbol lookup when parsing functions 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>