Assert in each AST constructor that symbols belong to the program of the parent.
Bug: tint:709
Change-Id: I82ae9b23c88e89714a44e057a0272f0293385aaf
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47624
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>