[outputfilemap] Fix another StringRef -> std::string conversion
[outputfilemap] Fix another StringRef -> std::string conversion

This fix affects master-next, but it's safe to put on master.

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