Leonard Pitts: Trump is a president whose words are weightless

Maybe you remember when Mexico was going to pay for the wall.You should. It was as recently as May. Donald Trump went to Nashville for one of those creepy rallies of his and renewed the vow he made ceaselessly during the 2016 campaign. Mexico, he promised, is "going to pay for the wall" along the U.S. southern border, "and they're going to enjoy it." The crowd cheered.It is probably useless to wonder what those cheering people might make of recent tweets wherein the selfsame Donald Trump threate