Fix _outConstraint() for "identity" constraints

Fix _outConstraint() for "identity" constraints

The set of fields printed by _outConstraint() in the CONSTR_IDENTITY
case didn't match the set of fields actually used in that case.  (The
code was probably uncarefully copied from the CONSTR_DEFAULT case.)
Fix that by using the right set of fields.  Since there is no read
support for this node type, this is really just for debugging output
right now, so it doesn't affect anything important.