SkSL: Allow 'const' on function parameters

Change-Id: I672345116e3b5538c0f7e8c5f2f74aa56bb81e6d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/372676
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed