C++17 compatibility: make WITH_TIDY=1 happy again.

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/runtime/entrypoints/quick/quick_jni_entrypoints.cc b/runtime/entrypoints/quick/quick_jni_entrypoints.cc
index 3c41a8c..5c86bbb 100644
--- a/runtime/entrypoints/quick/quick_jni_entrypoints.cc
+++ b/runtime/entrypoints/quick/quick_jni_entrypoints.cc
@@ -252,7 +252,7 @@
         return 0;
       default:
         LOG(FATAL) << "Unexpected return shorty character " << return_shorty_char;
-        return 0;
+        UNREACHABLE();
     }
   }
 }