[stdlib] replace an assign with a swap
[stdlib] replace an assign with a swap

Theoretically, this should cut down retain/release traffic

Swift SVN r22957

slavapestov/swift,airspeedswift/swift,gribozavr/swift,alblue/swift,JGiola/swift,benlangmuir/swift,russbishop/swift,glessard/swift,CodaFi/swift,ben-ng/swift,devincoughlin/swift,tardieu/swift,gregomni/swift,bitjammer/swift,LeoShimonaka/swift,uasys/swift,CodaFi/swift,deyton/swift,mightydeveloper/swift,khizkhiz/swift,tinysun212/swift-windows,apple/swift,gregomni/swift,tkremenek/swift,codestergit/swift,Jnosh/swift,djwbrown/swift,tkremenek/swift,calebd/swift,OscarSwanros/swift,Ivacker/swift,ben-ng/swift,tardieu/swift,sschiau/swift,huonw/swift,ahoppen/swift,cbrentharris/swift,swiftix/swift.old,brentdax/swift,tjw/swift,rudkx/swift,apple/swift,kusl/swift,gregomni/swift,felix91gr/swift,stephentyrone/swift,zisko/swift,allevato/swift,manavgabhawala/swift,tinysun212/swift-windows,jckarter/swift,kperryua/swift,therealbnut/swift,jtbandes/swift,natecook1000/swift,nathawes/swift,return/swift,manavgabhawala/swift,gribozavr/swift,jckarter/swift,jckarter/swift,emilstahl/swift,mightydeveloper/swift,kstaring/swift,johnno1962d/swift,johnno1962d/swift,harlanhaskins/swift,calebd/swift,jtbandes/swift,gribozavr/swift,IngmarStein/swift,djwbrown/swift,shahmishal/swift,austinzheng/swift,Ivacker/swift,hughbe/swift,parkera/swift,ahoppen/swift,milseman/swift,harlanhaskins/swift,modocache/swift,sschiau/swift,amraboelela/swift,ken0nek/swift,russbishop/swift,russbishop/swift,OscarSwanros/swift,practicalswift/swift,gregomni/swift,modocache/swift,xwu/swift,frootloops/swift,Jnosh/swift,arvedviehweger/swift,parkera/swift,nathawes/swift,brentdax/swift,stephentyrone/swift,ahoppen/swift,hughbe/swift,gmilos/swift,karwa/swift,parkera/swift,swiftix/swift,sschiau/swift,modocache/swift,LeoShimonaka/swift,jmgc/swift,CodaFi/swift,djwbrown/swift,benlangmuir/swift,austinzheng/swift,gottesmm/swift,johnno1962d/swift,tardieu/swift,gottesmm/swift,jopamer/swift,rudkx/swift,huonw/swift,gmilos/swift,JGiola/swift,lorentey/swift,aschwaighofer/swift,glessard/swift,cbrentharris/swift,xedin/swift,JaSpa/swift,devincoughlin/swift,hughbe/swift,swiftix/swift.old,parkera/swift,nathawes/swift,cbrentharris/swift,austinzheng/swift,austinzheng/swift,jopamer/swift,mightydeveloper/swift,kperryua/swift,SwiftAndroid/swift,jckarter/swift,stephentyrone/swift,jopamer/swift,slavapestov/swift,nathawes/swift,kusl/swift,slavapestov/swift,devincoughlin/swift,JGiola/swift,roambotics/swift,huonw/swift,shahmishal/swift,stephentyrone/swift,jckarter/swift,bitjammer/swift,Ivacker/swift,hooman/swift,lorentey/swift,uasys/swift,OscarSwanros/swift,frootloops/swift,frootloops/swift,shahmishal/swift,deyton/swift,shahmishal/swift,kperryua/swift,LeoShimonaka/swift,CodaFi/swift,slavapestov/swift,karwa/swift,aschwaighofer/swift,felix91gr/swift,jtbandes/swift,natecook1000/swift,OscarSwanros/swift,kentya6/swift,bitjammer/swift,practicalswift/swift,manavgabhawala/swift,felix91gr/swift,apple/swift,airspeedswift/swift,sdulal/swift,arvedviehweger/swift,jmgc/swift,SwiftAndroid/swift,CodaFi/swift,adrfer/swift,dduan/swift,SwiftAndroid/swift,IngmarStein/swift,sdulal/swift,adrfer/swift,CodaFi/swift,allevato/swift,SwiftAndroid/swift,tjw/swift,KrishMunot/swift,jtbandes/swift,apple/swift,JGiola/swift,natecook1000/swift,Jnosh/swift,xwu/swift,return/swift,devincoughlin/swift,sschiau/swift,ben-ng/swift,karwa/swift,adrfer/swift,gregomni/swift,cbrentharris/swift,Ivacker/swift,khizkhiz/swift,amraboelela/swift,huonw/swift,felix91gr/swift,tinysun212/swift-windows,IngmarStein/swift,kperryua/swift,OscarSwanros/swift,arvedviehweger/swift,zisko/swift,alblue/swift,dduan/swift,shajrawi/swift,jtbandes/swift,ahoppen/swift,Ivacker/swift,emilstahl/swift,khizkhiz/swift,JGiola/swift,Ivacker/swift,sdulal/swift,hughbe/swift,shahmishal/swift,shahmishal/swift,IngmarStein/swift,dduan/swift,roambotics/swift,bitjammer/swift,practicalswift/swift,arvedviehweger/swift,Ivacker/swift,kentya6/swift,ken0nek/swift,frootloops/swift,djwbrown/swift,ben-ng/swift,arvedviehweger/swift,IngmarStein/swift,jmgc/swift,kentya6/swift,xedin/swift,mightydeveloper/swift,jckarter/swift,manavgabhawala/swift,devincoughlin/swift,harlanhaskins/swift,lorentey/swift,deyton/swift,gmilos/swift,jtbandes/swift,gribozavr/swift,return/swift,tardieu/swift,johnno1962d/swift,tardieu/swift,JaSpa/swift,atrick/swift,russbishop/swift,jmgc/swift,modocache/swift,kstaring/swift,xwu/swift,karwa/swift,ken0nek/swift,atrick/swift,zisko/swift,practicalswift/swift,swiftix/swift.old,atrick/swift,huonw/swift,emilstahl/swift,russbishop/swift,shajrawi/swift,airspeedswift/swift,huonw/swift,LeoShimonaka/swift,glessard/swift,zisko/swift,zisko/swift,amraboelela/swift,MukeshKumarS/Swift,gmilos/swift,khizkhiz/swift,emilstahl/swift,sdulal/swift,JaSpa/swift,tjw/swift,atrick/swift,calebd/swift,benlangmuir/swift,kperryua/swift,kstaring/swift,xwu/swift,glessard/swift,LeoShimonaka/swift,kusl/swift,swiftix/swift.old,devincoughlin/swift,jckarter/swift,modocache/swift,kentya6/swift,mightydeveloper/swift,djwbrown/swift,milseman/swift,allevato/swift,hooman/swift,SwiftAndroid/swift,hooman/swift,JGiola/swift,modocache/sw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