Slightly updated DSL Switch API

PossibleExpression / PossibleStatement should only have been used in
cases where we do not have a position available.

Change-Id: I8cd3cafce21b3c18f03e75a0c822eea75c86f225
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/451896
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed