Before this change the implementation would ignore editions options set on
extension declarations.
This change also add much more coverage for valid messages.
Change-Id: I0e7029b1430a39859cdd4a93667ac20c7e0ff20e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/565396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Lasse Folger <lassefolger@google.com>