[loop-arc] Rename LoopRegionID => SubregionID. NFC.
[loop-arc] Rename LoopRegionID => SubregionID. NFC.

In order to handle non-local loop region edges, I will need to introduce
a SuccessorID type. Having a type named LoopRegionID and another type
named SuccessorID is confusing, so it makes sense to give LoopRegionID a
name that matches its use case.

Swift SVN r32643

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