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

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