[W]ith regard to enforcement actions at the Presidential level, certifications provided for under section 102(d)(3) [now section 716(d)(1)(C)] are intended to authorize the President and the Director of the Office of Management and Budget to preclude a suit by the Comptroller General against the President and his principal advisers and assistants, and against those units within the Executive Office of the President whose sole function is to advise and assist the President, for information which would not be available under the Freedom of Information Act.