[serialization] Fix "not implemented" check for cross-refs to extensions.
[serialization] Fix "not implemented" check for cross-refs to extensions.

These aren't implemented yet because there's no way to identify a particular
extension by name, but they shouldn't crash the module emitter.

Swift SVN r6178

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