Move function parsing code to ParseContext This change is pure refactoring. It will make it easier to fix bugs related to function declarations. BUG=angleproject:911 BUG=angleproject:1067 BUG=angleproject:1068 TEST=angle_unittests Change-Id: I031783dc02612d9cf3ff7a9c8291cf8ab33577aa Reviewed-on: https://chromium-review.googlesource.com/320081 Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>