ART: Add Clang's -Wused-but-marked-unused

Add detection of wrong unused annotations. Fix our codebase.

Change-Id: I85cc20f2eac71c1ec6c5c7cd6efb08454a629634
diff --git a/runtime/trace.cc b/runtime/trace.cc
index ea0a642..5375dc0 100644
--- a/runtime/trace.cc
+++ b/runtime/trace.cc
@@ -243,8 +243,7 @@
   the_trace->CompareAndUpdateStackTrace(thread, stack_trace);
 }
 
-static void ClearThreadStackTraceAndClockBase(Thread* thread ATTRIBUTE_UNUSED,
-                                              void* arg ATTRIBUTE_UNUSED) {
+static void ClearThreadStackTraceAndClockBase(Thread* thread, void* arg ATTRIBUTE_UNUSED) {
   thread->SetTraceClockBase(0);
   std::vector<mirror::ArtMethod*>* stack_trace = thread->GetStackTraceSample();
   thread->SetStackTraceSample(NULL);