Mythology surrounds Marie Antoinette, a French royal so removed from the struggles of common people that she allegedly couldn’t fathom why they were upset about not having bread. Sen. Elizabeth Warren (D-MA) plays a similar role in the debate over private equity in 401(k)s — and the irony is chocolate cake rich. Call her Sen. Antoinette. Warren tells average American workers, “Let them eat index funds,” while she herself indulges in higher-retur…