Add IRGenOptions flag to switch between pass managers.
Add IRGenOptions flag to switch between pass managers.

benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift,benlangmuir/swift,glessard/swift,JGiola/swift,glessard/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,JGiola/swift,apple/swift,roambotics/swift,apple/swift,benlangmuir/swift,apple/swift,roambotics/swift,JGiola/swift,apple/swift,JGiola/swift,roambotics/swift,JGiola/swift,glessard/swift,apple/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift