[libMigrator] Generalize CMake symlink to work on Windows
[libMigrator] Generalize CMake symlink to work on Windows

This fixes a bug found while building Swift for Windows. The CMake invocation
assumed that the host system had a utility called `create_symlink`. That
assumption breaks down on Windows, where `copy` is the equivalent tool.

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