Store builtin symbols as const pointers To do this we need two types of symbol table levels: A level for built-ins and a level for user-defined symbols. User-defined symbols are non-const because function symbols created based on function prototypes are changed when the function definition is parsed. On the other hand, we want to make built-in symbols constexpr, so we should only handle them through const pointers. This also gets rid of extra empty precision stack levels. Only one level is needed to store predefined precisions. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I9f14b24c2cfce272f22c16e7a8dfb653b849cbeb Reviewed-on: https://chromium-review.googlesource.com/892879 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>