Add const qualifiers to node inspection functions

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/nodes/list.c

src/backend/nodes/nodeFuncs.c

src/backend/nodes/outfuncs.c

src/backend/nodes/print.c

src/backend/optimizer/util/clauses.c

src/include/nodes/nodeFuncs.h

src/include/nodes/nodes.h

src/include/nodes/pg_list.h

src/include/nodes/print.h

src/include/optimizer/clauses.h