When encountering a type that does not have a MessageInfo, don't assume
that it's a legacy message that doesn't implement proto.Message. Add a
set of test messages exercising this case (panics prior to the
internal/impl change).
Change-Id: Ic1ec5ecfbe92278fbef44284ff52a0e0622a158c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/182477
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
