"The concerns about the fairness of such a determination would be heightened in the case of a sitting President, where a federal prosecutor's accusation of a crime... could carry consequences... beyond the realm of criminal justice. ...Even if an indictment were sealed during the President's term, OLC reasoned, "it would be very difficult to preserve [an indictment' s] secrecy," and if an indictment became public, "[t]he stigma and opprobrium" could imperil the President's ability to govern." ...[T]he possibility of the report's public disclosure and the absence of a neutral adjudicatory forum to review its findings counseled against potentially determining "that the person's conduct constitutes a federal offense.""