DROP TRANSFORM parsetree representation is no good

Running the regression tests with -DCOPY_PARSE_PLAN_TREES reveals that
DROP TRANSFORM parse trees fail to be copiable:

regression=# DROP TRANSFORM IF EXISTS FOR fake_type LANGUAGE plperl;
ERROR:  unrecognized node type: 1701866608

I've not tracked down the exact reason, but it kinda looks like the
"argument" field has been filled with a list of bare C strings, not
a list of String nodes which is what it'd have to be to work here.
        regards, tom lane