A clash of titans is shaping up in downtown Portland as Apple Inc. and Microsoft Corp. prepare to open retail outlets just a feet away from one another in Pioneer Place. Apple (NASDAQ: AAPL), the mall’s top performer on a sales-per-square-foot basis, is expanding its footprint by taking over space in the former Saks Fifth Avenue store. Whether Microsoft (NASDAQ: MFST) can attract similarly enthusiastic crowds to its new store, which will be located in the former Romano’s Macaroni Grill spot,…