Give SwitchCase it's own Statement::Kind value

Disambiguates Switch from SwitchCase, and makes is() and as() work.

Change-Id: I9b9b71dabd730247b20ec9b6df02b87e4978d534
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316577
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed