[Constraint system] Switch closure constraint generation to a visitor.
[Constraint system] Switch closure constraint generation to a visitor.

In preparation for implementing support for multiple-statement closure
inference, switch closure constraint generation over to a statement
visitor. Only the subset of statements that correspond to
single-expression closures is currently handled; there is no way to
enable this for multiple-statement closures yet.

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