[Driver] Always fall back to an arclite in the Xcode default toolchain (#22911)
[Driver] Always fall back to an arclite in the Xcode default toolchain (#22911)

...instead of the current toolchain, which we would have just
searched by looking relative to the swiftc binary.

rdar://problem/48044350
xedin/swift,hooman/swift,shajrawi/swift,stephentyrone/swift,devincoughlin/swift,karwa/swift,apple/swift,jckarter/swift,xedin/swift,apple/swift,xwu/swift,sschiau/swift,nathawes/swift,CodaFi/swift,stephentyrone/swift,stephentyrone/swift,nathawes/swift,glessard/swift,harlanhaskins/swift,apple/swift,rudkx/swift,hooman/swift,benlangmuir/swift,airspeedswift/swift,xedin/swift,parkera/swift,harlanhaskins/swift,xedin/swift,airspeedswift/swift,sschiau/swift,sschiau/swift,sschiau/swift,apple/swift,hooman/swift,sschiau/swift,devincoughlin/swift,airspeedswift/swift,CodaFi/swift,jmgc/swift,hooman/swift,gribozavr/swift,parkera/swift,jmgc/swift,lorentey/swift,aschwaighofer/swift,ahoppen/swift,harlanhaskins/swift,harlanhaskins/swift,CodaFi/swift,aschwaighofer/swift,shajrawi/swift,devincoughlin/swift,xwu/swift,glessard/swift,devincoughlin/swift,glessard/swift,aschwaighofer/swift,CodaFi/swift,rudkx/swift,hooman/swift,roambotics/swift,atrick/swift,gregomni/swift,nathawes/swift,gregomni/swift,xwu/swift,parkera/swift,shahmishal/swift,CodaFi/swift,harlanhaskins/swift,lorentey/swift,JGiola/swift,karwa/swift,aschwaighofer/swift,CodaFi/swift,shahmishal/swift,devincoughlin/swift,shajrawi/swift,jckarter/swift,allevato/swift,karwa/swift,jmgc/swift,gribozavr/swift,allevato/swift,shahmishal/swift,glessard/swift,rudkx/swift,tkremenek/swift,shajrawi/swift,glessard/swift,stephentyrone/swift,devincoughlin/swift,tkremenek/swift,allevato/swift,gribozavr/swift,aschwaighofer/swift,xedin/swift,JGiola/swift,allevato/swift,ahoppen/swift,airspeedswift/swift,shajrawi/swift,benlangmuir/swift,aschwaighofer/swift,allevato/swift,devincoughlin/swift,shahmishal/swift,jckarter/swift,ahoppen/swift,harlanhaskins/swift,rudkx/swift,lorentey/swift,roambotics/swift,CodaFi/swift,shahmishal/swift,karwa/swift,nathawes/swift,parkera/swift,jmgc/swift,allevato/swift,jckarter/swift,ahoppen/swift,atrick/swift,ahoppen/swift,rudkx/swift,jmgc/swift,JGiola/swift,tkremenek/swift,airspeedswift/swift,lorentey/swift,xedin/swift,lorentey/swift,JGiola/swift,glessard/swift,xwu/swift,apple/swift,stephentyrone/swift,benlangmuir/swift,airspeedswift/swift,gribozavr/swift,aschwaighofer/swift,hooman/swift,stephentyrone/swift,jckarter/swift,sschiau/swift,benlangmuir/swift,parkera/swift,tkremenek/swift,gribozavr/swift,atrick/swift,rudkx/swift,tkremenek/swift,sschiau/swift,parkera/swift,atrick/swift,airspeedswift/swift,nathawes/swift,gregomni/swift,roambotics/swift,harlanhaskins/swift,stephentyrone/swift,nathawes/swift,benlangmuir/swift,gribozavr/swift,nathawes/swift,gregomni/swift,shahmishal/swift,jmgc/swift,roambotics/swift,xwu/swift,karwa/swift,gribozavr/swift,shahmishal/swift,parkera/swift,roambotics/swift,lorentey/swift,tkremenek/swift,hooman/swift,sschiau/swift,apple/swift,devincoughlin/swift,JGiola/swift,shajrawi/swift,xedin/swift,atrick/swift,shajrawi/swift,allevato/swift,gregomni/swift,lorentey/swift,jckarter/swift,shajrawi/swift,xedin/swift,lorentey/swift,benlangmuir/swift,gribozavr/swift,shahmishal/swift,ahoppen/swift,parkera/swift,karwa/swift,roambotics/swift,xwu/swift,karwa/swift,atrick/swift,jckarter/swift,karwa/swift,gregomni/swift,jmgc/swift,xwu/swift,JGiola/swift,tkremenek/swift