GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/fields_illegal_tracing.txt

+In file included from fields_illegal_tracing.cpp:5:
+./fields_illegal_tracing.h:32:1: warning: [blink-gc] Class 'PartObject' contains invalid fields.
+class PartObject {
+^
+./fields_illegal_tracing.h:37:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
+    OwnPtr<HeapObject> m_obj1;
+    ^
+./fields_illegal_tracing.h:38:5: note: [blink-gc] RefPtr field 'm_obj2' to a GC managed class declared here:
+    RefPtr<HeapObject> m_obj2;
+    ^
+./fields_illegal_tracing.h:40:5: note: [blink-gc] std::unique_ptr field 'm_obj4' to a GC managed class declared here:
+    std::unique_ptr<HeapObject> m_obj4;
+    ^
+./fields_illegal_tracing.h:43:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
+class HeapObject : public GarbageCollectedFinalized<HeapObject> {
+^
+./fields_illegal_tracing.h:48:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
+    OwnPtr<HeapObject> m_obj1;
+    ^
+./fields_illegal_tracing.h:49:5: note: [blink-gc] RefPtr field 'm_obj2' to a GC managed class declared here:
+    RefPtr<HeapObject> m_obj2;
+    ^
+./fields_illegal_tracing.h:51:5: note: [blink-gc] std::unique_ptr field 'm_obj4' to a GC managed class declared here:
+    std::unique_ptr<HeapObject> m_obj4;
+    ^
+fields_illegal_tracing.cpp:9:1: warning: [blink-gc] Class 'PartObject' has untraced or not traceable fields.
+void PartObject::trace(Visitor* visitor) {
+^
+./fields_illegal_tracing.h:37:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
+    OwnPtr<HeapObject> m_obj1;
+    ^
+./fields_illegal_tracing.h:38:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
+    RefPtr<HeapObject> m_obj2;
+    ^
+./fields_illegal_tracing.h:40:5: note: [blink-gc] Untraceable field 'm_obj4' declared here:
+    std::unique_ptr<HeapObject> m_obj4;
+    ^
+fields_illegal_tracing.cpp:16:1: warning: [blink-gc] Class 'HeapObject' has untraced or not traceable fields.
+void HeapObject::trace(Visitor* visitor) {
+^
+./fields_illegal_tracing.h:48:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
+    OwnPtr<HeapObject> m_obj1;
+    ^
+./fields_illegal_tracing.h:49:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
+    RefPtr<HeapObject> m_obj2;
+    ^
+./fields_illegal_tracing.h:51:5: note: [blink-gc] Untraceable field 'm_obj4' declared here:
+    std::unique_ptr<HeapObject> m_obj4;
+    ^
+4 warnings generated.