Add const qualification to symbol accesses All accesses to built-in symbols now happen through const-qualified pointers. This also encapsulates TSymbolTableLevel inside TSymbolTable. This prepares for statically allocating built-in symbols. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I473014d978daa765b4a733d761d6c08b28288776 Reviewed-on: https://chromium-review.googlesource.com/859959 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>