LOUISVILLE, Ky. — The challenge finally arrived for Rory McIlroy, and he was better than ever Sunday to win the PGA Championship. On a back nine filled with clutch shots and as much tension as a major can provide, McIlroy emerged from a four-man race to outlast Phil Mickelson and the darkness at Valhalla to capture his second straight major. McIlroy closed with a 3-under 68 and became only the fourth player in the last century of golf to win four majors at 25 or younger.