Move symbol-table push/pop methods out of IRGenerator.

This will allow us to use AutoSymbolTable in more places (and helps cut
down IRGenerator, as we transition away from it).

Change-Id: Ie88c149e0f7eee080a8a4c8cbbc34c8a2a5401d1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449756
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed