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

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/compiler/optimizing/code_generator.cc b/compiler/optimizing/code_generator.cc
index c2e83cd..2184f99 100644
--- a/compiler/optimizing/code_generator.cc
+++ b/compiler/optimizing/code_generator.cc
@@ -1662,7 +1662,7 @@
     case 3: return kQuickAllocArrayResolved64;
   }
   LOG(FATAL) << "Unreachable";
-  return kQuickAllocArrayResolved;
+  UNREACHABLE();
 }
 
 }  // namespace art