[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.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5433,7 +5433,7 @@ class BindingProducer {
   /// This is useful to be able to exhaustively attempt bindings
   /// for type variables found at one level, before proceeding to
   /// supertypes or literal defaults etc.
-  virtual bool needsToComputeNext() const { return false; }
+  virtual bool needsToComputeNext() const = 0;
 };
 
 class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
@@ -5592,6 +5592,8 @@ class DisjunctionChoiceProducer : public BindingProducer<DisjunctionChoice> {
                              IsExplicitConversion, isBeginningOfPartition);
   }
 
+  bool needsToComputeNext() const override { return false; }
+
 private:
   // Partition the choices in the disjunction into groups that we will
   // iterate over in an order appropriate to attempt to stop before we