[stdlib] NFC: Fix deprecation warning related to `index(of:)`
[stdlib] NFC: Fix deprecation warning related to `index(of:)`

`index(of:)` has been replaced with `firstIndex(of:)`.

devincoughlin/swift,amraboelela/swift,practicalswift/swift,sschiau/swift,xwu/swift,gribozavr/swift,devincoughlin/swift,amraboelela/swift,tkremenek/swift,rudkx/swift,nathawes/swift,airspeedswift/swift,CodaFi/swift,stephentyrone/swift,atrick/swift,rudkx/swift,amraboelela/swift,jmgc/swift,gribozavr/swift,practicalswift/swift,rudkx/swift,atrick/swift,jmgc/swift,karwa/swift,xwu/swift,airspeedswift/swift,benlangmuir/swift,jmgc/swift,alblue/swift,allevato/swift,shahmishal/swift,stephentyrone/swift,atrick/swift,sschiau/swift,CodaFi/swift,allevato/swift,parkera/swift,ahoppen/swift,jckarter/swift,roambotics/swift,glessard/swift,stephentyrone/swift,natecook1000/swift,lorentey/swift,brentdax/swift,jckarter/swift,sschiau/swift,devincoughlin/swift,apple/swift,sschiau/swift,sschiau/swift,gregomni/swift,gribozavr/swift,alblue/swift,austinzheng/swift,xedin/swift,JGiola/swift,amraboelela/swift,glessard/swift,jmgc/swift,harlanhaskins/swift,shahmishal/swift,shahmishal/swift,jckarter/swift,apple/swift,jmgc/swift,harlanhaskins/swift,danielmartin/swift,practicalswift/swift,xwu/swift,airspeedswift/swift,allevato/swift,karwa/swift,lorentey/swift,nathawes/swift,brentdax/swift,karwa/swift,devincoughlin/swift,ahoppen/swift,devincoughlin/swift,tkremenek/swift,allevato/swift,atrick/swift,austinzheng/swift,hooman/swift,parkera/swift,practicalswift/swift,aschwaighofer/swift,shajrawi/swift,rudkx/swift,rudkx/swift,parkera/swift,gregomni/swift,practicalswift/swift,glessard/swift,gribozavr/swift,natecook1000/swift,harlanhaskins/swift,alblue/swift,jckarter/swift,karwa/swift,amraboelela/swift,shahmishal/swift,xwu/swift,tkremenek/swift,natecook1000/swift,aschwaighofer/swift,parkera/swift,airspeedswift/swift,ahoppen/swift,allevato/swift,jckarter/swift,gribozavr/swift,tkremenek/swift,gribozavr/swift,roambotics/swift,lorentey/swift,airspeedswift/swift,hooman/swift,practicalswift/swift,nathawes/swift,roambotics/swift,devincoughlin/swift,gregomni/swift,apple/swift,benlangmuir/swift,airspeedswift/swift,benlangmuir/swift,aschwaighofer/swift,ahoppen/swift,glessard/swift,shahmishal/swift,shajrawi/swift,amraboelela/swift,tkremenek/swift,lorentey/swift,nathawes/swift,jckarter/swift,lorentey/swift,gribozavr/swift,roambotics/swift,gregomni/swift,harlanhaskins/swift,harlanhaskins/swift,atrick/swift,austinzheng/swift,roambotics/swift,lorentey/swift,sschiau/swift,apple/swift,roambotics/swift,practicalswift/swift,jmgc/swift,rudkx/swift,shajrawi/swift,brentdax/swift,austinzheng/swift,danielmartin/swift,xwu/swift,danielmartin/swift,stephentyrone/swift,devincoughlin/swift,alblue/swift,brentdax/swift,nathawes/swift,karwa/swift,apple/swift,gribozavr/swift,CodaFi/swift,tkremenek/swift,gregomni/swift,xwu/swift,xedin/swift,aschwaighofer/swift,xedin/swift,CodaFi/swift,stephentyrone/swift,harlanhaskins/swift,parkera/swift,hooman/swift,hooman/swift,parkera/swift,brentdax/swift,allevato/swift,shajrawi/swift,JGiola/swift,JGiola/swift,shahmishal/swift,alblue/swift,shahmishal/swift,brentdax/swift,ahoppen/swift,atrick/swift,danielmartin/swift,JGiola/swift,brentdax/swift,shajrawi/swift,CodaFi/swift,nathawes/swift,austinzheng/swift,parkera/swift,natecook1000/swift,sschiau/swift,xwu/swift,stephentyrone/swift,sschiau/swift,danielmartin/swift,apple/swift,ahoppen/swift,hooman/swift,gregomni/swift,glessard/swift,jckarter/swift,natecook1000/swift,CodaFi/swift,danielmartin/swift,aschwaighofer/swift,lorentey/swift,allevato/swift,danielmartin/swift,hooman/swift,natecook1000/swift,alblue/swift,amraboelela/swift,jmgc/swift,JGiola/swift,aschwaighofer/swift,airspeedswift/swift,devincoughlin/swift,stephentyrone/swift,karwa/swift,benlangmuir/swift,karwa/swift,nathawes/swift,tkremenek/swift,xedin/swift,benlangmuir/swift,xedin/swift,CodaFi/swift,aschwaighofer/swift,harlanhaskins/swift,shajrawi/swift,hooman/swift,xedin/swift,shahmishal/swift,austinzheng/swift,shajrawi/swift,benlangmuir/swift,shajrawi/swift,karwa/swift,practicalswift/swift,parkera/swift,natecook1000/swift,alblue/swift,xedin/swift,JGiola/swift,austinzheng/swift,xedin/swift,lorentey/swift,glessard/swift