moved SkSL ModifiersDeclaration data into IRNode

Change-Id: Iebe59b32a965e383d2b032258d7731645e0bd1a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325622
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
8 files changed