[siloptimizer] Use .str() on StringRef.
[siloptimizer] Use .str() on StringRef.

llvm recently removed the conversion operator that would convert
StringRef into str() implictly.  Now we have to be more explicit.
See: https://github.com/llvm/llvm-project/commit/adcd02683856c30ba6f349279509acecd90063df

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