Cycles: Make Clang 3.6 happy with const debug flags reference

For some reason it did not like the explicit const qualifier for a
custom type.

intern/cycles/util/util_debug.cpp

@@ -114,7 +114,7 @@ void DebugFlags::reset()
 }
 
 std::ostream& operator <<(std::ostream &os,
-                          const DebugFlagsRef debug_flags)
+                          DebugFlagsConstRef debug_flags)
 {
 	os << "CPU flags:\n"
 	   << "  AVX2   : " << string_from_bool(debug_flags.cpu.avx2)  << "\n"

intern/cycles/util/util_debug.h

@@ -125,13 +125,14 @@ private:
 };
 
 typedef DebugFlags& DebugFlagsRef;
+typedef const DebugFlags& DebugFlagsConstRef;
 
 inline DebugFlags& DebugFlags() {
   return DebugFlags::get();
 }
 
 std::ostream& operator <<(std::ostream &os,
-                          const DebugFlagsRef debug_flags);
+                          DebugFlagsConstRef debug_flags);
 
 CCL_NAMESPACE_END