Sema: Avoid copying BindingSets

They're stored inside the ConstraintGraphNode now, so returning a
`const BindingSet *` from within determineBestBindings() should
be safe.

include/swift/Sema/CSBindings.h

@@ -385,6 +385,10 @@ public:
   BindingSet(ConstraintSystem &CS, TypeVariableType *TypeVar,
              const PotentialBindings &info);
 
+  BindingSet(BindingSet &&other) = default;
+
+  BindingSet(const BindingSet &other) = delete;
+
   ConstraintSystem &getConstraintSystem() const { return CS; }
 
   TypeVariableType *getTypeVariable() const { return TypeVar; }

include/swift/Sema/ConstraintSystem.h

@@ -5270,7 +5270,7 @@ public:
 
   /// Determine whether given type variable with its set of bindings is viable
   /// to be attempted on the next step of the solver.
-  std::optional<BindingSet> determineBestBindings(
+  const BindingSet *determineBestBindings(
       llvm::function_ref<void(const BindingSet &)> onCandidate);
 
   /// Get bindings for the given type variable based on current
@@ -6200,7 +6200,7 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
 public:
   using Element = TypeVariableBinding;
 
-  TypeVarBindingProducer(BindingSet &bindings);
+  TypeVarBindingProducer(const BindingSet &bindings);
 
   /// Retrieve a set of bindings available in the current state.
   ArrayRef<Binding> getCurrentBindings() const { return Bindings; }