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

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/runtime/common_runtime_test.cc b/runtime/common_runtime_test.cc
index fcf6bb2..7388c2e 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -139,7 +139,7 @@
   PreRuntimeCreate();
   if (!Runtime::Create(options, false)) {
     LOG(FATAL) << "Failed to create runtime";
-    return;
+    UNREACHABLE();
   }
   PostRuntimeCreate();
   runtime_.reset(Runtime::Current());