[Frontend] Fix use of unique pointer to VFS
[Frontend] Fix use of unique pointer to VFS

rdar://72137092

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