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>