Sema: Only use PreparedOverload::Change::AddedBindConstraint if the locator didn't change

For a Bind constraint generated by a same-type requirement, we must
preserve the locator so we need to record it like any other kind of
constraint.

This fixes a diagnostic regression with -solver-enable-prepared-overloads.

Author: slavapestov

include/swift/Sema/PreparedOverload.h

@@ -104,9 +104,10 @@ struct PreparedOverloadChange {
     /// For ChangeKind::AddedConstraint.
     Constraint *TheConstraint;
 
+    /// For ChangeKind::AddedBindConstraint.
     struct {
       TypeBase *FirstType;
-      TypeBase * SecondType;
+      TypeBase *SecondType;
     } Bind;
 
     /// For ChangeKind::OpenedTypes.
@@ -187,6 +188,10 @@ class PreparedOverload final :
 
 struct PreparedOverloadBuilder {
   SmallVector<PreparedOverload::Change, 8> Changes;
+  ConstraintLocator *Locator;
+
+  PreparedOverloadBuilder(ConstraintLocator *locator)
+    : Locator(locator) {}
 
   void addedTypeVariable(TypeVariableType *typeVar) {
     PreparedOverload::Change change;

lib/Sema/CSSimplify.cpp

@@ -16529,14 +16529,22 @@ void ConstraintSystem::addConstraint(ConstraintKind kind, Type first,
                                      PreparedOverloadBuilder *preparedOverload) {
   if (preparedOverload) {
     ASSERT(PreparingOverload);
-    if (kind == ConstraintKind::Bind) {
+
+    auto *locatorPtr = getConstraintLocator(locator);
+
+    // Fast path to save on memory usage.
+    if (kind == ConstraintKind::Bind &&
+        locatorPtr == preparedOverload->Locator) {
       ASSERT(!isFavored);
       preparedOverload->addedBindConstraint(first, second);
       return;
     }
-    auto c = Constraint::create(*this, kind, first, second,
-                                getConstraintLocator(locator));
-    if (isFavored) c->setFavored();
+
+    auto c = Constraint::create(*this, kind, first, second, locatorPtr);
+
+    if (isFavored)
+      c->setFavored();
+
     preparedOverload->addedConstraint(c);
     return;
   }

lib/Sema/TypeOfReference.cpp

@@ -2811,7 +2811,8 @@ void ConstraintSystem::replayChanges(
       addConstraint(ConstraintKind::Bind,
                     change.Bind.FirstType,
                     change.Bind.SecondType,
-                    locator, /*isFavored=*/false);
+                    locator,
+                    /*isFavored=*/false);
       break;
 
     case PreparedOverload::Change::OpenedTypes: {
@@ -2890,7 +2891,7 @@ PreparedOverload *ConstraintSystem::prepareOverload(OverloadChoice choice,
   ASSERT(!PreparingOverload);
   PreparingOverload = true;
 
-  PreparedOverloadBuilder builder;
+  PreparedOverloadBuilder builder(locator);
   Type openedType;
   Type thrownErrorType;
   std::tie(openedType, thrownErrorType) = prepareOverloadImpl(