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

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/runtime/transaction.cc b/runtime/transaction.cc
index e4bf447..62482fd 100644
--- a/runtime/transaction.cc
+++ b/runtime/transaction.cc
@@ -531,7 +531,7 @@
       break;
     default:
       LOG(FATAL) << "Unknown value kind " << static_cast<int>(field_value.kind);
-      break;
+      UNREACHABLE();
   }
 }
 
@@ -558,7 +558,7 @@
           break;
         default:
           LOG(FATAL) << "Unknown interned string kind";
-          break;
+          UNREACHABLE();
       }
       break;
     }
@@ -572,13 +572,13 @@
           break;
         default:
           LOG(FATAL) << "Unknown interned string kind";
-          break;
+          UNREACHABLE();
       }
       break;
     }
     default:
       LOG(FATAL) << "Unknown interned string op";
-      break;
+      UNREACHABLE();
   }
 }
 
@@ -669,9 +669,10 @@
       break;
     case Primitive::kPrimNot:
       LOG(FATAL) << "ObjectArray should be treated as Object";
-      break;
+      UNREACHABLE();
     default:
       LOG(FATAL) << "Unsupported type " << array_type;
+      UNREACHABLE();
   }
 }