gen_builtin_symbols.py now generates code for initializing built-in
variable symbols as well. Some of the variable symbols are static, but
some of them also get initialized dynamically based on values in
ShBuiltInResources.

The static symbols have get functions in a header file so they can be
referenced from AST traversers as well without doing a lookup.

BUG=angleproject:2267
TEST=angle_unittests, angle_end2end_tests

Change-Id: Ida7f3aeb06d2bce0f737f1483b1bd5833aeddd2e
Reviewed-on: https://chromium-review.googlesource.com/911768Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>