SkSL: Ensure struct declarations contain at least one member

Bug: skia:11314
Change-Id: I66476543462ae378a5bfb6cbd902dfa2f5fc45f5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369917
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
7 files changed