qapi: Improve reporting of redefinition

Point to the previous definition, unless it's a built-in.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-26-armbru@redhat.com>
4 files changed