Serialization: Implement LocalDeclTableInfo::GetExternalKey()
Serialization: Implement LocalDeclTableInfo::GetExternalKey()

This allows iterating over serialized decl tables, which is useful
for debugging.

parkera/swift,CodaFi/swift,jckarter/swift,rudkx/swift,ahoppen/swift,jmgc/swift,airspeedswift/swift,glessard/swift,hooman/swift,xwu/swift,rudkx/swift,stephentyrone/swift,stephentyrone/swift,atrick/swift,tkremenek/swift,harlanhaskins/swift,tkremenek/swift,jmgc/swift,aschwaighofer/swift,allevato/swift,airspeedswift/swift,tkremenek/swift,allevato/swift,airspeedswift/swift,benlangmuir/swift,aschwaighofer/swift,rudkx/swift,ahoppen/swift,benlangmuir/swift,xwu/swift,gregomni/swift,parkera/swift,gregomni/swift,atrick/swift,gregomni/swift,airspeedswift/swift,rudkx/swift,benlangmuir/swift,tkremenek/swift,xwu/swift,atrick/swift,aschwaighofer/swift,harlanhaskins/swift,jmgc/swift,tkremenek/swift,glessard/swift,xwu/swift,parkera/swift,ahoppen/swift,atrick/swift,hooman/swift,jmgc/swift,atrick/swift,glessard/swift,ahoppen/swift,jckarter/swift,parkera/swift,airspeedswift/swift,ahoppen/swift,roambotics/swift,stephentyrone/swift,jckarter/swift,tkremenek/swift,jckarter/swift,jmgc/swift,apple/swift,JGiola/swift,roambotics/swift,xwu/swift,nathawes/swift,tkremenek/swift,benlangmuir/swift,harlanhaskins/swift,aschwaighofer/swift,jmgc/swift,nathawes/swift,JGiola/swift,CodaFi/swift,nathawes/swift,nathawes/swift,gregomni/swift,aschwaighofer/swift,allevato/swift,CodaFi/swift,CodaFi/swift,JGiola/swift,glessard/swift,CodaFi/swift,JGiola/swift,xwu/swift,JGiola/swift,hooman/swift,parkera/swift,apple/swift,airspeedswift/swift,hooman/swift,airspeedswift/swift,stephentyrone/swift,jckarter/swift,gregomni/swift,stephentyrone/swift,jckarter/swift,nathawes/swift,hooman/swift,roambotics/swift,rudkx/swift,CodaFi/swift,allevato/swift,nathawes/swift,parkera/swift,roambotics/swift,harlanhaskins/swift,hooman/swift,harlanhaskins/swift,allevato/swift,atrick/swift,jckarter/swift,gregomni/swift,apple/swift,nathawes/swift,parkera/swift,hooman/swift,JGiola/swift,ahoppen/swift,benlangmuir/swift,jmgc/swift,apple/swift,harlanhaskins/swift,stephentyrone/swift,harlanhaskins/swift,roambotics/swift,allevato/swift,benlangmuir/swift,glessard/swift,stephentyrone/swift,CodaFi/swift,glessard/swift,xwu/swift,apple/swift,apple/swift,aschwaighofer/swift,allevato/swift,parkera/swift,aschwaighofer/swift,roambotics/swift,rudkx/swift