AST: ProtocolConformanceRef::subst() returns an invalid conformance instead of trapping
AST: ProtocolConformanceRef::subst() returns an invalid conformance instead of trapping

This is for consistency with Type::subst() which returns ErrorTypes.

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