Sema: Bring back BindingSet::operator bool but call it BindingSet::isViable()

include/swift/Sema/CSBindings.h

@@ -482,6 +482,12 @@ public:
            !Defaults.empty();
   }
 
+  /// Determine whether this set can be chosen as the next binding set
+  /// to attempt.
+  bool isViable() const {
+    return hasViableBindings() || isDirectHole();
+  }
+
   ArrayRef<Constraint *> getConformanceRequirements() const {
     return Protocols;
   }

lib/Sema/CSBindings.cpp

@@ -1170,7 +1170,7 @@ std::optional<BindingSet> ConstraintSystem::determineBestBindings(
     // associated with given type variable, any default constraints,
     // or any conformance requirements to literal protocols with can
     // produce a default type.
-    bool isViable = bindings.hasViableBindings() || bindings.isDirectHole();
+    bool isViable = bindings.isViable();
 
     bindings.inferTransitiveSupertypeBindings();
     bindings.determineLiteralCoverage();

lib/Sema/ConstraintGraph.cpp

@@ -922,7 +922,7 @@ bool ConstraintGraph::contractEdges() {
     if (tyvar1->getImpl().canBindToInOut()) {
       bool isNotContractable = true;
       auto bindings = CS.getBindingsFor(tyvar1);
-      if (bindings.hasViableBindings() || bindings.isDirectHole()) {
+      if (bindings.isViable()) {
         // Holes can't be contracted.
         if (bindings.isHole())
           continue;