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>