Sema: Add BindingSet::operator==

This will be used for debugging.

include/swift/Sema/CSBindings.h

@@ -588,44 +588,11 @@ public:
 
   static BindingScore formBindingScore(const BindingSet &b);
 
-  /// Compare two sets of bindings, where \c x < y indicates that
-  /// \c x is a better set of bindings that \c y.
-  friend bool operator<(const BindingSet &x, const BindingSet &y) {
-    auto xScore = formBindingScore(x);
-    auto yScore = formBindingScore(y);
+  bool operator==(const BindingSet &other);
 
-    if (xScore < yScore)
-      return true;
-
-    if (yScore < xScore)
-      return false;
-
-    auto xDefaults = x.getNumViableDefaultableBindings();
-    auto yDefaults = y.getNumViableDefaultableBindings();
-
-    // If there is a difference in number of default types,
-    // prioritize bindings with fewer of them.
-    if (xDefaults != yDefaults)
-      return xDefaults < yDefaults;
-
-    // If neither type variable is a "hole" let's check whether
-    // there is a subtype relationship between them and prefer
-    // type variable which represents superclass first in order
-    // for "subtype" type variable to attempt more bindings later.
-    // This is required because algorithm can't currently infer
-    // bindings for subtype transitively through superclass ones.
-    if (!(std::get<0>(xScore) && std::get<0>(yScore))) {
-      if (x.Info.isSubtypeOf(y.getTypeVariable()))
-        return false;
-
-      if (y.Info.isSubtypeOf(x.getTypeVariable()))
-        return true;
-    }
-
-    // As a last resort, let's check if the bindings are
-    // potentially incomplete, and if so, let's de-prioritize them.
-    return x.isPotentiallyIncomplete() < y.isPotentiallyIncomplete();
-  }
+  /// Compare two sets of bindings, where \c this < other indicates that
+  /// \c this is a better set of bindings that \c other.
+  bool operator<(const BindingSet &other);
 
   void dump(llvm::raw_ostream &out, unsigned indent) const;