Use darwinpcs for ARM64.
Use darwinpcs for ARM64.

The clang TargetInfo for ARM64 defaults to aapcs unless darwinpcs is
specifically requested (which clang's driver does when compiling for
ARM64), so we end up with the wrong ABI.

I filed <rdar://problem/16687086> because it seems reasonable for
TargetInfo to default to darwinpcs when the target OS is known to be
isOSDarwin.

Swift SVN r16654

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