Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/class_linker.cc b/runtime/class_linker.cc
index f6717fb..854effd 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -5284,6 +5284,7 @@
       break;
     default:
       LOG(FATAL) << "Unreachable - invocation type: " << type;
+      UNREACHABLE();
   }
   if (resolved == nullptr) {
     // Search by name, which works across dex files.