Don't bother to attach column name lists to RowExprs of named types.

If a RowExpr is marked as returning a named composite type, we aren't
going to consult its colnames list; we'll use the attribute names
shown for the type in pg_attribute.  Hence, skip storing that list,
to save a few nanoseconds when copying the expression tree around.

Discussion: https://postgr.es/m/2950001.1638729947@sss.pgh.pa.us

src/backend/optimizer/util/var.c

@@ -809,6 +809,7 @@ flatten_join_alias_vars_mutator(Node *node,
 			rowexpr->args = fields;
 			rowexpr->row_typeid = var->vartype;
 			rowexpr->row_format = COERCE_IMPLICIT_CAST;
+			/* vartype will always be RECORDOID, so we always need colnames */
 			rowexpr->colnames = colnames;
 			rowexpr->location = var->location;