[Sema] Account for ExistentialType in lookupExistentialConformance.
[Sema] Account for ExistentialType in lookupExistentialConformance.

JGiola/swift,glessard/swift,roambotics/swift,roambotics/swift,rudkx/swift,gregomni/swift,JGiola/swift,glessard/swift,roambotics/swift,JGiola/swift,apple/swift,gregomni/swift,ahoppen/swift,glessard/swift,ahoppen/swift,atrick/swift,roambotics/swift,JGiola/swift,gregomni/swift,apple/swift,rudkx/swift,apple/swift,atrick/swift,gregomni/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,rudkx/swift,rudkx/swift,atrick/swift,benlangmuir/swift,glessard/swift,JGiola/swift,atrick/swift,atrick/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,gregomni/swift,apple/swift,glessard/swift,JGiola/swift,glessard/swift,apple/swift,gregomni/swift,rudkx/swift,rudkx/swift,benlangmuir/swift,atrick/swift,apple/swift,benlangmuir/swift,ahoppen/swift,ahoppen/swift