[Conformance checking] Skip potential witnesses with error types in them.
[Conformance checking] Skip potential witnesses with error types in them.

This fixes several crasher regressions from the flattening of
SubstitutionMap.

danielmartin/swift,aschwaighofer/swift,shahmishal/swift,ahoppen/swift,glessard/swift,jckarter/swift,amraboelela/swift,shahmishal/swift,rudkx/swift,nathawes/swift,roambotics/swift,amraboelela/swift,shahmishal/swift,CodaFi/swift,brentdax/swift,sschiau/swift,CodaFi/swift,jopamer/swift,austinzheng/swift,natecook1000/swift,jckarter/swift,jckarter/swift,gregomni/swift,airspeedswift/swift,danielmartin/swift,jmgc/swift,glessard/swift,devincoughlin/swift,amraboelela/swift,rudkx/swift,aschwaighofer/swift,xwu/swift,ahoppen/swift,stephentyrone/swift,CodaFi/swift,rudkx/swift,jmgc/swift,apple/swift,shajrawi/swift,airspeedswift/swift,shajrawi/swift,parkera/swift,shahmishal/swift,karwa/swift,nathawes/swift,devincoughlin/swift,tkremenek/swift,jopamer/swift,apple/swift,xedin/swift,nathawes/swift,CodaFi/swift,jckarter/swift,JGiola/swift,CodaFi/swift,aschwaighofer/swift,xwu/swift,gregomni/swift,gregomni/swift,jmgc/swift,hooman/swift,gribozavr/swift,JGiola/swift,benlangmuir/swift,sschiau/swift,huonw/swift,harlanhaskins/swift,huonw/swift,jopamer/swift,karwa/swift,karwa/swift,hooman/swift,danielmartin/swift,shahmishal/swift,stephentyrone/swift,harlanhaskins/swift,austinzheng/swift,brentdax/swift,karwa/swift,jmgc/swift,stephentyrone/swift,lorentey/swift,airspeedswift/swift,lorentey/swift,stephentyrone/swift,tkremenek/swift,harlanhaskins/swift,karwa/swift,atrick/swift,jopamer/swift,alblue/swift,karwa/swift,brentdax/swift,benlangmuir/swift,jmgc/swift,ahoppen/swift,huonw/swift,jckarter/swift,karwa/swift,xwu/swift,ahoppen/swift,atrick/swift,brentdax/swift,natecook1000/swift,tkremenek/swift,alblue/swift,glessard/swift,roambotics/swift,parkera/swift,parkera/swift,brentdax/swift,practicalswift/swift,xwu/swift,brentdax/swift,aschwaighofer/swift,rudkx/swift,natecook1000/swift,devincoughlin/swift,jmgc/swift,shajrawi/swift,gregomni/swift,parkera/swift,practicalswift/swift,lorentey/swift,JGiola/swift,rudkx/swift,allevato/swift,lorentey/swift,apple/swift,alblue/swift,roambotics/swift,tkremenek/swift,stephentyrone/swift,stephentyrone/swift,danielmartin/swift,aschwaighofer/swift,karwa/swift,gribozavr/swift,jopamer/swift,xedin/swift,shajrawi/swift,nathawes/swift,lorentey/swift,amraboelela/swift,danielmartin/swift,airspeedswift/swift,austinzheng/swift,xedin/swift,apple/swift,hooman/swift,jopamer/swift,roambotics/swift,amraboelela/swift,shahmishal/swift,glessard/swift,austinzheng/swift,huonw/swift,jckarter/swift,natecook1000/swift,natecook1000/swift,glessard/swift,amraboelela/swift,danielmartin/swift,nathawes/swift,jckarter/swift,alblue/swift,parkera/swift,atrick/swift,jopamer/swift,devincoughlin/swift,benlangmuir/swift,harlanhaskins/swift,rudkx/swift,brentdax/swift,xwu/swift,hooman/swift,hooman/swift,natecook1000/swift,harlanhaskins/swift,alblue/swift,gribozavr/swift,austinzheng/swift,hooman/swift,airspeedswift/swift,tkremenek/swift,shajrawi/swift,xwu/swift,atrick/swift,sschiau/swift,shajrawi/swift,JGiola/swift,xwu/swift,JGiola/swift,harlanhaskins/swift,harlanhaskins/swift,practicalswift/swift,shahmishal/swift,JGiola/swift,austinzheng/swift,benlangmuir/swift,sschiau/swift,practicalswift/swift,apple/swift,allevato/swift,roambotics/swift,gribozavr/swift,allevato/swift,parkera/swift,austinzheng/swift,roambotics/swift,gribozavr/swift,nathawes/swift,tkremenek/swift,tkremenek/swift,huonw/swift,xedin/swift,huonw/swift,nathawes/swift,atrick/swift,amraboelela/swift,atrick/swift,shajrawi/swift,devincoughlin/swift,devincoughlin/swift,danielmartin/swift,practicalswift/swift,benlangmuir/swift,hooman/swift,practicalswift/swift,CodaFi/swift,sschiau/swift,lorentey/swift,shajrawi/swift,sschiau/swift,airspeedswift/swift,xedin/swift,sschiau/swift,devincoughlin/swift,ahoppen/swift,alblue/swift,xedin/swift,practicalswift/swift,benlangmuir/swift,jmgc/swift,shahmishal/swift,glessard/swift,stephentyrone/swift,CodaFi/swift,allevato/swift,huonw/swift,allevato/swift,gribozavr/swift,allevato/swift,airspeedswift/swift,practicalswift/swift,gribozavr/swift,xedin/swift,parkera/swift,gregomni/swift,natecook1000/swift,ahoppen/swift,aschwaighofer/swift,apple/swift,gribozavr/swift,aschwaighofer/swift,allevato/swift,lorentey/swift,lorentey/swift,gregomni/swift,sschiau/swift,alblue/swift,xedin/swift,parkera/swift,devincoughlin/swift