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/compiler/optimizing/constant_folding.cc b/compiler/optimizing/constant_folding.cc
index 10a7e46..fca9933 100644
--- a/compiler/optimizing/constant_folding.cc
+++ b/compiler/optimizing/constant_folding.cc
@@ -28,9 +28,9 @@
     // Traverse this block's instructions in (forward) order and
     // replace the ones that can be statically evaluated by a
     // compile-time counterpart.
-    for (HInstructionIterator it(block->GetInstructions());
-         !it.Done(); it.Advance()) {
-      HInstruction* inst = it.Current();
+    for (HInstructionIterator inst_it(block->GetInstructions());
+         !inst_it.Done(); inst_it.Advance()) {
+      HInstruction* inst = inst_it.Current();
       if (inst->IsBinaryOperation()) {
         // Constant folding: replace `op(a, b)' with a constant at
         // compile time if `a' and `b' are both constants.