[ConstraintSystem] NFC: Require all binding producers to implement `needsToComputeNext`
[ConstraintSystem] NFC: Require all binding producers to implement `needsToComputeNext`

Instead of defaulting to `false`, let's make implementers of `BindingProducer`
consider what needs to happen.

glessard/swift,hooman/swift,apple/swift,apple/swift,parkera/swift,xwu/swift,atrick/swift,hooman/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,JGiola/swift,parkera/swift,xwu/swift,ahoppen/swift,tkremenek/swift,gregomni/swift,atrick/swift,hooman/swift,ahoppen/swift,benlangmuir/swift,rudkx/swift,gregomni/swift,hooman/swift,xwu/swift,hooman/swift,glessard/swift,roambotics/swift,tkremenek/swift,roambotics/swift,JGiola/swift,xwu/swift,tkremenek/swift,tkremenek/swift,tkremenek/swift,apple/swift,benlangmuir/swift,gregomni/swift,benlangmuir/swift,ahoppen/swift,hooman/swift,glessard/swift,atrick/swift,xwu/swift,glessard/swift,parkera/swift,gregomni/swift,rudkx/swift,apple/swift,parkera/swift,parkera/swift,glessard/swift,roambotics/swift,ahoppen/swift,ahoppen/swift,atrick/swift,rudkx/swift,glessard/swift,apple/swift,hooman/swift,JGiola/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,xwu/swift,roambotics/swift,parkera/swift,JGiola/swift,gregomni/swift,rudkx/swift,parkera/swift,atrick/swift,gregomni/swift,roambotics/swift,ahoppen/swift,atrick/swift,tkremenek/swift,tkremenek/swift,rudkx/swift,parkera/swift,JGiola/swift,rudkx/swift,xwu/swift