GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt

Index: tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt
diff --git a/tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt b/tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt
index ed0dbcae5a8c5b078689719d0a7a1e108cbd6cf1..fa6b9f544a0e6443c8a6ebdefc4866f7e7026774 100644
--- a/tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt
+++ b/tools/clang/blink_gc_plugin/tests/templated_class_with_local_class_requires_trace.txt
@@ -17,4 +17,10 @@ class TemplatedObject final
 ./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.