We have a dozen PQset*() functions.  PQresultSetInstanceData() and this
were the libpq setter functions having a different word order.  Adopt
the majority word order.

Reviewed by Alvaro Herrera and Robert Haas, though this choice of name
was not unanimous.

Discussion: https://postgr.es/m/20210605060555.GA216695@rfd.leadboat.com