(S corporation shareholder can increase basis through bona fide indebtedness; general federal tax principles to be used to determine bona fideness rather than "actual economic outlay" test; "circular loans" can be bona fide, based on facts and circumstances; guarantees do not constitute bona fide indebtedness unless shareholder actually makes payment; no basis increase for contribution of unsecured demand promissory note; effective upon publishing of regulations).