TSymbolTable API can be cleaned up further now that we have separate
logic for inserting builtins and user-defined symbols.

BUG=angleprojec:2267
TEST=angle_unittests

Change-Id: I7a228891ecdf4696e50868b9e7dfc2a50b5d8e92
Reviewed-on: https://chromium-review.googlesource.com/941301
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>