When loading a *MessageInfo for a legacy message type, check to see if
the Go type contains at least one field which looks like a message
field. Specifically, look for at least one field with a `protobuf:` tag,
or an XXX_unrecognized field.
If a message has no recognizable fields, assume that it's something we
don't know how to interpret and treat it as an aberrant message.
Change-Id: If5c09087f1a0187271c98539d761395a2ee70a9e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/210617
Reviewed-by: Joe Tsai <joetsai@google.com>