ART: More warnings

Enable -Wno-conversion-null, -Wredundant-decls and -Wshadow in general,
and -Wunused-but-set-parameter for GCC builds.

Change-Id: I81bbdd762213444673c65d85edae594a523836e5
diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index bfaa2bb..012f9f9 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -143,7 +143,7 @@
   soft_reference_queue_.ClearWhiteReferences(&cleared_references_, is_marked_callback, arg);
   weak_reference_queue_.ClearWhiteReferences(&cleared_references_, is_marked_callback, arg);
   {
-    TimingLogger::ScopedTiming t(concurrent ? "EnqueueFinalizerReferences" :
+    TimingLogger::ScopedTiming t2(concurrent ? "EnqueueFinalizerReferences" :
         "(Paused)EnqueueFinalizerReferences", timings);
     if (concurrent) {
       StartPreservingReferences(self);