Improved SkSL array size parsing

We previously had no way to signal a parse error from arraySize,
resulting in a cascade of additional errors downstream. This tightens
up the behavior and allows us to fail more gracefully.

Bug: skia:12416
Change-Id: I83d3d5bc1dc63395edb325297375a6eb52415817
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/512952
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed