qapi: Fix crash on redefinition with a different condition

qapi: Fix crash on redefinition with a different condition

QAPISchema._make_implicit_object_type() asserts that when an implicit
object type is used multiple times, @ifcond is the same for all uses.
It will be for legitimate uses, i.e. simple union branch wrapper
types.  A comment explains this.

The assertion fails when a command or event is redefined with a
different condition.  The redefinition is an error, but it's flagged
only later.

Fixing the assertion would complicate matters further.  Not
worthwhile, drop it instead.  We really need to get rid of simple
unions.

Tweak test case redefined-event to cover redefinition with a different
condition.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210806120510.2367124-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>