[serialization] Mark some transient types as llvm_unreachable.
[serialization] Mark some transient types as llvm_unreachable.

Also, remove a defensive check for a case there is now support for.

Swift SVN r6099

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