Add a SkSLSwitchCase cpp file.

Previously, all the logic was crammed into the header. Now the
header contains the getters, and the cpp contains the non-trivial
methods.

Change-Id: I3ec65cb42cc4add44924efede16a8d5f297e34aa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/709636
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
5 files changed