GC plugin: have -Werror issue errors for inputs having only warnings.

tools/clang/blink_gc_plugin/tests/raw_ptr_to_gc_managed_class_error.txt

+In file included from raw_ptr_to_gc_managed_class_error.cpp:5:
+./raw_ptr_to_gc_managed_class_error.h:14:1: error: [blink-gc] Class 'PartObject' contains invalid fields.
+class PartObject {
+^
+./raw_ptr_to_gc_managed_class_error.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_error.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_error.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_error.h:24:1: error: [blink-gc] Class 'HeapObject' contains invalid fields.
+class HeapObject : public GarbageCollected<HeapObject> {
+^
+./raw_ptr_to_gc_managed_class_error.h:29:5: note: [blink-gc] Raw pointer field 'm_objs' to a GC managed class declared here:
+    HeapVector<HeapObject*> m_objs;
+    ^
+2 errors generated.