GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt

 In file included from templated_class_with_local_class_requires_trace.cpp:5:
 ./templated_class_with_local_class_requires_trace.h:22:1: warning: [blink-gc] Class 'TemplatedObject<blink::HeapObject>' contains invalid fields.
 class TemplatedObject final
 ^
 ./templated_class_with_local_class_requires_trace.h:46:5: note: [blink-gc] OwnPtr field 'm_ownRef' to a GC managed class declared here:
     OwnPtr<T> m_ownRef;
     ^
 ./templated_class_with_local_class_requires_trace.h:32:5: warning: [blink-gc] Class 'Local' contains invalid fields.
     class Local final : public GarbageCollected<Local> {
     ^
 ./templated_class_with_local_class_requires_trace.h:41:9: note: [blink-gc] OwnPtr field 'm_object' to a GC managed class declared here:
         OwnPtr<HeapObject> m_object;
         ^
 ./templated_class_with_local_class_requires_trace.h:32:5: warning: [blink-gc] Class 'Local' requires finalization.
     class Local final : public GarbageCollected<Local> {
     ^
 ./templated_class_with_local_class_requires_trace.h:41:9: note: [blink-gc] Field 'm_object' requiring finalization declared here:
         OwnPtr<HeapObject> m_object;
         ^
-3 warnings generated.
+./templated_class_with_local_class_requires_trace.h:34:9: warning: [blink-gc] Class 'Local' has untraced or not traceable fields.
+        void trace(Visitor* visitor)
+        ^
+./templated_class_with_local_class_requires_trace.h:41:9: note: [blink-gc] Untraceable field 'm_object' declared here:
+        OwnPtr<HeapObject> m_object;
+        ^
+4 warnings generated.