SkSL vars now track their declaration instead of their initial value

This should not cause any functional changes, and is just a prerequisite
for upcoming DSL work.

Change-Id: Iea165d3b7ede39ccc9cf5f5d78f623bc883b391e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356816
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
10 files changed