Use musttail when merging swifttail calls.
Use musttail when merging swifttail calls.

Fixes rdar://75899459.

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