[build-script] When building compiler-rt, follow LLVM's lead by specifying that we are using LLVM's external project mechanism.
[build-script] When building compiler-rt, follow LLVM's lead by specifying that we are using LLVM's external project mechanism.

This matches the LLVM build more closely preventing potential build issues
coming up.

rdar://26284108

return/swift,uasys/swift,felix91gr/swift,karwa/swift,manavgabhawala/swift,gribozavr/swift,gottesmm/swift,ahoppen/swift,jopamer/swift,codestergit/swift,jckarter/swift,lorentey/swift,brentdax/swift,xwu/swift,swiftix/swift,tkremenek/swift,rudkx/swift,gribozavr/swift,practicalswift/swift,austinzheng/swift,stephentyrone/swift,hughbe/swift,hooman/swift,manavgabhawala/swift,calebd/swift,atrick/swift,ahoppen/swift,uasys/swift,return/swift,sschiau/swift,natecook1000/swift,OscarSwanros/swift,ben-ng/swift,kperryua/swift,kperryua/swift,jtbandes/swift,tinysun212/swift-windows,tardieu/swift,ahoppen/swift,swiftix/swift,sschiau/swift,zisko/swift,calebd/swift,JGiola/swift,parkera/swift,deyton/swift,harlanhaskins/swift,arvedviehweger/swift,kperryua/swift,austinzheng/swift,milseman/swift,Jnosh/swift,roambotics/swift,ken0nek/swift,milseman/swift,gregomni/swift,nathawes/swift,harlanhaskins/swift,xedin/swift,harlanhaskins/swift,shahmishal/swift,aschwaighofer/swift,roambotics/swift,nathawes/swift,sschiau/swift,kstaring/swift,lorentey/swift,huonw/swift,natecook1000/swift,tardieu/swift,hughbe/swift,parkera/swift,jckarter/swift,russbishop/swift,devincoughlin/swift,zisko/swift,hughbe/swift,tkremenek/swift,milseman/swift,amraboelela/swift,russbishop/swift,rudkx/swift,JaSpa/swift,xwu/swift,allevato/swift,airspeedswift/swift,alblue/swift,benlangmuir/swift,ben-ng/swift,ken0nek/swift,ahoppen/swift,deyton/swift,felix91gr/swift,danielmartin/swift,OscarSwanros/swift,gribozavr/swift,uasys/swift,manavgabhawala/swift,brentdax/swift,airspeedswift/swift,shajrawi/swift,swiftix/swift,IngmarStein/swift,gribozavr/swift,arvedviehweger/swift,lorentey/swift,ahoppen/swift,brentdax/swift,zisko/swift,deyton/swift,alblue/swift,gottesmm/swift,milseman/swift,bitjammer/swift,devincoughlin/swift,harlanhaskins/swift,hooman/swift,tkremenek/swift,karwa/swift,jckarter/swift,tardieu/swift,jopamer/swift,Jnosh/swift,karwa/swift,shajrawi/swift,kstaring/swift,deyton/swift,alblue/swift,devincoughlin/swift,devincoughlin/swift,shajrawi/swift,tjw/swift,therealbnut/swift,roambotics/swift,codestergit/swift,therealbnut/swift,therealbnut/swift,gottesmm/swift,djwbrown/swift,xedin/swift,aschwaighofer/swift,ben-ng/swift,harlanhaskins/swift,bitjammer/swift,huonw/swift,aschwaighofer/swift,gmilos/swift,felix91gr/swift,uasys/swift,gottesmm/swift,rudkx/swift,dreamsxin/swift,zisko/swift,jopamer/swift,arvedviehweger/swift,gottesmm/swift,jopamer/swift,danielmartin/swift,CodaFi/swift,brentdax/swift,benlangmuir/swift,deyton/swift,codestergit/swift,brentdax/swift,bitjammer/swift,jtbandes/swift,ben-ng/swift,brentdax/swift,stephentyrone/swift,glessard/swift,xwu/swift,CodaFi/swift,stephentyrone/swift,allevato/swift,kperryua/swift,hughbe/swift,ken0nek/swift,jopamer/swift,sschiau/swift,jtbandes/swift,return/swift,hughbe/swift,calebd/swift,manavgabhawala/swift,hooman/swift,frootloops/swift,parkera/swift,xwu/swift,austinzheng/swift,modocache/swift,glessard/swift,tinysun212/swift-windows,codestergit/swift,aschwaighofer/swift,xwu/swift,therealbnut/swift,parkera/swift,tardieu/swift,djwbrown/swift,russbishop/swift,xwu/swift,atrick/swift,tjw/swift,djwbrown/swift,jmgc/swift,swiftix/swift,shajrawi/swift,hooman/swift,jtbandes/swift,return/swift,danielmartin/swift,swiftix/swift,tinysun212/swift-windows,felix91gr/swift,hooman/swift,ben-ng/swift,rudkx/swift,allevato/swift,gregomni/swift,frootloops/swift,manavgabhawala/swift,tardieu/swift,practicalswift/swift,xedin/swift,harlanhaskins/swift,atrick/swift,atrick/swift,practicalswift/swift,xedin/swift,shahmishal/swift,tjw/swift,allevato/swift,austinzheng/swift,russbishop/swift,gregomni/swift,nathawes/swift,amraboelela/swift,apple/swift,gribozavr/swift,apple/swift,tardieu/swift,JaSpa/swift,apple/swift,sschiau/swift,frootloops/swift,austinzheng/swift,hughbe/swift,zisko/swift,karwa/swift,djwbrown/swift,shahmishal/swift,tkremenek/swift,zisko/swift,kstaring/swift,calebd/swift,hughbe/swift,huonw/swift,russbishop/swift,OscarSwanros/swift,gottesmm/swift,jmgc/swift,natecook1000/swift,gregomni/swift,arvedviehweger/swift,lorentey/swift,Jnosh/swift,jopamer/swift,nathawes/swift,IngmarStein/swift,frootloops/swift,jckarter/swift,huonw/swift,IngmarStein/swift,ken0nek/swift,allevato/swift,gribozavr/swift,roambotics/swift,shahmishal/swift,xwu/swift,ken0nek/swift,shajrawi/swift,modocache/swift,bitjammer/swift,ben-ng/swift,JGiola/swift,karwa/swift,felix91gr/swift,nathawes/swift,ken0nek/swift,milseman/swift,kperryua/swift,IngmarStein/swift,therealbnut/swift,karwa/swift,sschiau/swift,jmgc/swift,lorentey/swift,devincoughlin/swift,gregomni/swift,karwa/swift,calebd/swift,shajrawi/swift,return/swift,JaSpa/swift,milseman/swift,practicalswift/swift,arvedviehweger/swift,tinysun212/swift-windows,huonw/swift,JGiola/swift,hooman/swift,kstaring/swift,deyton/swift,natecook1000/swift,gmilos/swift,aschwaighofer/swift,tjw/swift,parkera/swift,austinzheng/swift,danielmartin/swift,Jnosh/swift,JaSpa/swift,airspeedswift/swift,amraboelela/swift,dreamsxin/swift,djwbrown/swift,calebd/swift,IngmarStein/swift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