"The proposed rule ... bars employers from offering this type of HRA side-by-side with a traditional health plan; it requires employers to offer this type of HRA on the same terms to all similarly situated employees; and it bars employers from using this type of HRA to subsidize short-term, limited duration coverage.... [A]bsent these safeguards, the Administration's HRA proposal would cause very large increases in individual market premiums, although the precise magnitude of those increases is uncertain."