[Runtime] Fix calling convention of async function.
[Runtime] Fix calling convention of async function.

Otherwise, we run into a type error on turning on swiftasync.

roambotics/swift,ahoppen/swift,atrick/swift,xwu/swift,benlangmuir/swift,gregomni/swift,glessard/swift,parkera/swift,hooman/swift,ahoppen/swift,JGiola/swift,tkremenek/swift,glessard/swift,gregomni/swift,JGiola/swift,apple/swift,apple/swift,xwu/swift,rudkx/swift,JGiola/swift,ahoppen/swift,gregomni/swift,benlangmuir/swift,atrick/swift,tkremenek/swift,benlangmuir/swift,atrick/swift,ahoppen/swift,roambotics/swift,hooman/swift,xwu/swift,atrick/swift,parkera/swift,glessard/swift,rudkx/swift,apple/swift,apple/swift,tkremenek/swift,JGiola/swift,hooman/swift,roambotics/swift,gregomni/swift,parkera/swift,roambotics/swift,xwu/swift,parkera/swift,parkera/swift,atrick/swift,JGiola/swift,benlangmuir/swift,hooman/swift,rudkx/swift,tkremenek/swift,tkremenek/swift,JGiola/swift,rudkx/swift,apple/swift,benlangmuir/swift,hooman/swift,tkremenek/swift,benlangmuir/swift,xwu/swift,rudkx/swift,ahoppen/swift,apple/swift,rudkx/swift,hooman/swift,glessard/swift,glessard/swift,gregomni/swift,ahoppen/swift,hooman/swift,glessard/swift,roambotics/swift,atrick/swift,roambotics/swift,gregomni/swift,xwu/swift,parkera/swift,parkera/swift,xwu/swift,parkera/swift,tkremenek/swift