Instead of using an std::map at each symbol table level, use the
gen_builtin_symbols.py script to build a function to query unmangled
names.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I4f1cf1df1f50fe9d909f3249150ee002ee6efb61
Reviewed-on: https://chromium-review.googlesource.com/931885
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>