[Statement checker] Localize 'fallthrough' checking of case variable bindings.
[Statement checker] Localize 'fallthrough' checking of case variable bindings.

Rather than track the "last fallthrough statement" in the statement
checker to check case bindings while processing a 'switch' statement,
use the fallthrough source/destination already computed to check this
condition in the fallthrough statement itself. Eliminates yet more
state from the statement type checker.

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