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/jni_internal.cc b/runtime/jni_internal.cc
index dd66af7..67e52cb 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -196,8 +196,8 @@
     // Failed to find type from the signature of the field.
     DCHECK(soa.Self()->IsExceptionPending());
     ThrowLocation throw_location;
-    StackHandleScope<1> hs(soa.Self());
-    Handle<mirror::Throwable> cause(hs.NewHandle(soa.Self()->GetException(&throw_location)));
+    StackHandleScope<1> hs2(soa.Self());
+    Handle<mirror::Throwable> cause(hs2.NewHandle(soa.Self()->GetException(&throw_location)));
     soa.Self()->ClearException();
     std::string temp;
     soa.Self()->ThrowNewExceptionF(throw_location, "Ljava/lang/NoSuchFieldError;",