[Runtime] Immediate release and return when destroying partial instance of pure ObjC class.

Make swift_deallocPartialClassInstance check if the object's class is a pure ObjC class, in which case there are no ivar destroyers and we can just return immediately.

It's possible for an allocWithZone: override to cause self to be a special object constructed in read-only memory. swift_deallocPartialClassInstance calls object_setClass to avoid running the dealloc method of any Swift subclasses, but this call crashes if self is read-only. It's unnecessary when the object's class is pure ObjC and therefore there are no Swift subclasses, so just skip it entirely.

rdar://107756747

Author: mikeash

stdlib/public/runtime/HeapObject.cpp

@@ -849,6 +849,17 @@ void swift::swift_deallocPartialClassInstance(HeapObject *object,
   // Destroy ivars
   auto *classMetadata = _swift_getClassOfAllocated(object)->getClassObject();
   assert(classMetadata && "Not a class?");
+
+#if SWIFT_OBJC_INTEROP
+  // If the object's class is already pure ObjC class, just release it and move
+  // on. There are no ivar destroyers. This avoids attempting to mutate
+  // placeholder objects statically created in read-only memory.
+  if (classMetadata->isPureObjC()) {
+    objc_release((id)object);
+    return;
+  }
+#endif
+
   while (classMetadata != metadata) {
 #if SWIFT_OBJC_INTEROP
     // If we have hit a pure Objective-C class, we won't see another ivar