AST: Slim down ASTContext::getOverrideGenericSignature()
AST: Slim down ASTContext::getOverrideGenericSignature()

We don't actually have to store the superclass type or the interface
type to be unique enough; just the derived class declaration alone
is sufficient.

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