GC plugin: consider references equal to raw pointers.

tools/clang/blink_gc_plugin/tests/raw_ptr_to_gc_managed_class.txt

 In file included from raw_ptr_to_gc_managed_class.cpp:5:
 ./raw_ptr_to_gc_managed_class.h:14:1: warning: [blink-gc] Class 'PartObject' contains invalid fields.
 class PartObject {
 ^
-./raw_ptr_to_gc_managed_class.h:17:5: note: [blink-gc] Raw pointer field 'm_obj' to a GC managed class declared here:
+./raw_ptr_to_gc_managed_class.h:19:5: note: [blink-gc] Raw pointer field 'm_obj' to a GC managed class declared here:
     RawPtr<HeapObject> m_obj;
     ^
-./raw_ptr_to_gc_managed_class.h:20:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
+./raw_ptr_to_gc_managed_class.h:20:5: note: [blink-gc] Raw pointer field 'm_rawObj' to a GC managed class declared here:
+    HeapObject* m_rawObj;
+    ^
+./raw_ptr_to_gc_managed_class.h:21:5: note: [blink-gc] Reference pointer field 'm_refObj' to a GC managed class declared here:
+    HeapObject& m_refObj;
+    ^
+./raw_ptr_to_gc_managed_class.h:24:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
 class HeapObject : public GarbageCollected<HeapObject> {
 ^
-./raw_ptr_to_gc_managed_class.h:25:5: note: [blink-gc] Raw pointer field 'm_objs' to a GC managed class declared here:
+./raw_ptr_to_gc_managed_class.h:29:5: note: [blink-gc] Raw pointer field 'm_objs' to a GC managed class declared here:
     HeapVector<HeapObject*> m_objs;
     ^
 2 warnings generated.