"Two recent opinions from the U.S. Courts of Appeals forecast a troubling expansion of the imposition of withdrawal liability of non-signatory entities under the successor liability doctrine. While the imposition of such liability is not new, recent case law extends the doctrine beyond the reach of the Seventh Circuit ... to which it had previously been confined, and appears to ease the requirements for a claim against a putative successor. These cases also illustrate the effect of successor liability on the 'building and construction industry exception' to withdrawal liability."