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/collector/semi_space.cc b/runtime/gc/collector/semi_space.cc
index e141b6f..cb9f111 100644
--- a/runtime/gc/collector/semi_space.cc
+++ b/runtime/gc/collector/semi_space.cc
@@ -224,7 +224,7 @@
   // Need to do this before the checkpoint since we don't want any threads to add references to
   // the live stack during the recursive mark.
   if (kUseThreadLocalAllocationStack) {
-    TimingLogger::ScopedTiming t("RevokeAllThreadLocalAllocationStacks", GetTimings());
+    TimingLogger::ScopedTiming t2("RevokeAllThreadLocalAllocationStacks", GetTimings());
     heap_->RevokeAllThreadLocalAllocationStacks(self_);
   }
   heap_->SwapStacks(self_);
@@ -368,7 +368,7 @@
   CHECK_EQ(is_large_object_space_immune_, collect_from_space_only_);
   space::LargeObjectSpace* los = GetHeap()->GetLargeObjectsSpace();
   if (is_large_object_space_immune_ && los != nullptr) {
-    TimingLogger::ScopedTiming t("VisitLargeObjects", GetTimings());
+    TimingLogger::ScopedTiming t2("VisitLargeObjects", GetTimings());
     DCHECK(collect_from_space_only_);
     // Delay copying the live set to the marked set until here from
     // BindBitmaps() as the large objects on the allocation stack may