"[A] loan program could protect participant benefits by helping distressed multiemployer plans return to sound financial footing.... The most direct way for Congress to improve the financial strength of the PBGC would be to increase the current premium level.... Another way to increase funding to PBGC's multiemployer program is to deduct premium payments from benefit payments made to participants in all plans covered by the program.... The Committee could also consider whether the PBGC should have broader authority to proactively restructure distressed plans to enable them to remain solvent.... [T]he Committee might consider whether accrued plan benefits should become adjustable after restructuring."