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/interpreter/interpreter.cc b/runtime/interpreter/interpreter.cc
index 44e2029..18de133 100644
--- a/runtime/interpreter/interpreter.cc
+++ b/runtime/interpreter/interpreter.cc
@@ -315,7 +315,7 @@
   kSwitchImpl,            // Switch-based interpreter implementation.
   kComputedGotoImplKind   // Computed-goto-based interpreter implementation.
 };
-std::ostream& operator<<(std::ostream& os, const InterpreterImplKind& rhs) {
+static std::ostream& operator<<(std::ostream& os, const InterpreterImplKind& rhs) {
   os << ((rhs == kSwitchImpl) ? "Switch-based interpreter" : "Computed-goto-based interpreter");
   return os;
 }