moved SkSL VarDeclarations data into IRNode

Change-Id: I03bdef43c79bc3c997f9a9a6aa8fbb1a7194943a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326437
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
19 files changed