Style cleanup for ObjPtr<>-ify changes.

Replace "ObjPtr<.> const" with "const ObjPtr<.>".

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: I5a1c080bc88b091e15ee9eb0bb1ef6f6f290701c
diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index 95871da..1c751e1 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -181,7 +181,7 @@
   if (UNLIKELY(IsEmpty())) {
     return;
   }
-  ObjPtr<mirror::Reference> const head = list_;
+  const ObjPtr<mirror::Reference> head = list_;
   ObjPtr<mirror::Reference> ref = head;
   do {
     mirror::HeapReference<mirror::Object>* referent_addr = ref->GetReferentReferenceAddr();