Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index 83da063..ad3bb11 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -813,6 +813,7 @@
           break;
         default:
           LOG(FATAL) << "Unreachable";
+          UNREACHABLE();
         }
         TimingLogger::ScopedTiming t(name, GetTimings());
         ScanObjectVisitor visitor(this);