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

Add detection of wrong unused annotations. Fix our codebase.

Change-Id: I85cc20f2eac71c1ec6c5c7cd6efb08454a629634
diff --git a/compiler/utils/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index a894319..3e8d9c3 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -683,7 +683,7 @@
 
 bool Thumb2Assembler::Is32BitDataProcessing(Condition cond ATTRIBUTE_UNUSED,
                                             Opcode opcode,
-                                            bool set_cc ATTRIBUTE_UNUSED,
+                                            bool set_cc,
                                             Register rn,
                                             Register rd,
                                             const ShifterOperand& so) {