Update fast dealloc to match libobjc

Author: Catfish-Man

include/swift/Runtime/HeapObject.h

@@ -1120,13 +1120,9 @@ swift_getTypeName(const Metadata *type, bool qualified);
   auto heapObj = reinterpret_cast<HeapObject *>(self); \
   heapObj->refCounts.setPureSwiftDeallocation(false); \
 } \
-- (bool)_setAssociatedObject:(id)obj \
-                      forKey:(const void *)key \
-           associationPolicy:(objc_AssociationPolicy)policy { \
+- (void)_noteAssociatedObjects { \
   auto heapObj = reinterpret_cast<HeapObject *>(self); \
   heapObj->refCounts.setPureSwiftDeallocation(false); \
-  /* false to let libobjc know it still needs to associate the object */ \
-  return false; \
 } \
 - (void)dealloc { \
   swift_rootObjCDealloc(reinterpret_cast<HeapObject *>(self)); \

stdlib/public/runtime/HeapObject.cpp

@@ -612,9 +612,7 @@ void swift::swift_rootObjCDealloc(HeapObject *self) {
 
 #if SWIFT_OBJC_INTEROP
 static bool _check_fast_dealloc() {
-  //This will always be in libobjc, so RTLD_DEFAULT won't have to do an
-  //expensive search in practice
-  return dlsym(RTLD_DEFAULT, "_objc_has_weak_formation_callout") != nullptr;
+  return dlsym(RTLD_NEXT, "_objc_has_weak_formation_callout") != nullptr;
 }
 #endif