build: simplify macOS path (NFC)
build: simplify macOS path (NFC)

It does not make sense to use the `cl` frontend to clang to build for
macOS.  Remove the unnecessary condition and always perform the
operation.

jckarter/swift,glessard/swift,hooman/swift,xwu/swift,roambotics/swift,atrick/swift,benlangmuir/swift,stephentyrone/swift,gregomni/swift,JGiola/swift,hooman/swift,tkremenek/swift,stephentyrone/swift,ahoppen/swift,atrick/swift,hooman/swift,apple/swift,airspeedswift/swift,parkera/swift,rudkx/swift,allevato/swift,airspeedswift/swift,CodaFi/swift,allevato/swift,atrick/swift,benlangmuir/swift,CodaFi/swift,hooman/swift,ahoppen/swift,roambotics/swift,atrick/swift,ahoppen/swift,gregomni/swift,JGiola/swift,stephentyrone/swift,jmgc/swift,jckarter/swift,xwu/swift,hooman/swift,ahoppen/swift,parkera/swift,nathawes/swift,jmgc/swift,rudkx/swift,parkera/swift,xwu/swift,rudkx/swift,nathawes/swift,glessard/swift,nathawes/swift,stephentyrone/swift,tkremenek/swift,nathawes/swift,gregomni/swift,allevato/swift,airspeedswift/swift,JGiola/swift,benlangmuir/swift,nathawes/swift,tkremenek/swift,gregomni/swift,tkremenek/swift,apple/swift,rudkx/swift,nathawes/swift,JGiola/swift,jckarter/swift,CodaFi/swift,tkremenek/swift,CodaFi/swift,tkremenek/swift,allevato/swift,airspeedswift/swift,CodaFi/swift,benlangmuir/swift,roambotics/swift,parkera/swift,JGiola/swift,xwu/swift,ahoppen/swift,roambotics/swift,xwu/swift,jmgc/swift,allevato/swift,jmgc/swift,glessard/swift,atrick/swift,allevato/swift,benlangmuir/swift,jmgc/swift,rudkx/swift,roambotics/swift,stephentyrone/swift,parkera/swift,stephentyrone/swift,airspeedswift/swift,xwu/swift,jckarter/swift,jmgc/swift,ahoppen/swift,CodaFi/swift,rudkx/swift,allevato/swift,jckarter/swift,benlangmuir/swift,parkera/swift,nathawes/swift,airspeedswift/swift,stephentyrone/swift,apple/swift,airspeedswift/swift,hooman/swift,jmgc/swift,apple/swift,apple/swift,parkera/swift,apple/swift,hooman/swift,jckarter/swift,atrick/swift,xwu/swift,jckarter/swift,CodaFi/swift,JGiola/swift,gregomni/swift,roambotics/swift,glessard/swift,tkremenek/swift,parkera/swift,gregomni/swift,glessard/swift,glessard/swift