[stdlib] Limit array casts to Array<T>
[stdlib] Limit array casts to Array<T>

There's no need to support up- or down-casts for NativeArray<T> or
Slice<T>.  Slice might turn out to be easy, but we don't have time right
now.  Doing it for NativeArray<T> would have efficiency costs that we
don't want to pay.

Swift SVN r17323

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