Clang style plugin: add warn-only option and use it on Windows

tools/clang/plugins/Options.h

Index: tools/clang/plugins/Options.h
diff --git a/tools/clang/plugins/Options.h b/tools/clang/plugins/Options.h
index 112ec1054fde3d0e5cca9c870cb1959fc7251321..5131905f2b7e4f543fe94775cce0823cf32d66b3 100644
--- a/tools/clang/plugins/Options.h
+++ b/tools/clang/plugins/Options.h
@@ -12,12 +12,14 @@ struct Options {
       : check_base_classes(false),
         check_enum_last_value(false),
         with_ast_visitor(false),
-        check_templates(false) {}
+        check_templates(false),
+        warn_only(false) {}
 
   bool check_base_classes;
   bool check_enum_last_value;
   bool with_ast_visitor;
   bool check_templates;
+  bool warn_only;
 };
 
 }  // namespace chrome_checker