IRGen: Fix a non null terminated string error
IRGen: Fix a non null terminated string error

Just use the StringRef as index to a string map. Using data() will fail
if the StringRef does not contain a null terminated string.

The problem is that we get the data of a StringRef (which is not null
terminated) to intialize a StringRef with char* which expects a null
terminated string.

Found by a ASAN bot.

rdar://56340563

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