[CSBindings] Couple of small PotentialBindings cleanups

- Use `int` instead of `unsigned int` to represent # of non-defaultable bindings.
  Because the formula is `-(Total - Defaultable)` which could only be `0` or negative.

- Increase number of defaultable bindings iff the binding is accepted.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -177,6 +177,9 @@ void ConstraintSystem::PotentialBindings::addPotentialBinding(
   if (!isViable(binding))
     return;
 
+  if (binding.isDefaultableBinding())
+    ++NumDefaultableBindings;
+
   Bindings.push_back(std::move(binding));
 }
 
@@ -660,7 +663,6 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
     if (!exactTypes.insert(type->getCanonicalType()).second)
       continue;
 
-    ++result.NumDefaultableBindings;
     result.addPotentialBinding({type, AllowedBindingKind::Exact,
                                 constraint->getKind(), nullptr,
                                 constraint->getLocator()});

lib/Sema/ConstraintSystem.h

@@ -2816,8 +2816,7 @@ class ConstraintSystem {
   };
 
   struct PotentialBindings {
-    using BindingScore =
-        std::tuple<bool, bool, bool, bool, unsigned char, unsigned int>;
+    using BindingScore = std::tuple<bool, bool, bool, bool, unsigned char, int>;
 
     TypeVariableType *TypeVar;