Add DSL PossibleExpression & PossibleStatement

These are currently unused, but in future CLs they will be used to
capture line number information in DSL error handling.

Change-Id: Ieee730e0ad8323043437972fedb5bec471c367e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375069
Reviewed-by: John Stiles <johnstiles@google.com>
9 files changed