Fix bitfield enums to not trigger upcoming Clang warning

src/messages.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The infrastructure used for (localized) message reporting in V8.
 //
 // Note: there's a big unresolved issue about ownership of the data
 // structures used by this framework.
 
 #ifndef V8_MESSAGES_H_
@@ skipping 631 matching lines @@
   T(DataCloneErrorSharedArrayBufferNotTransferred,                             \
     "A SharedArrayBuffer could not be cloned. SharedArrayBuffer must be "      \
     "transferred.")                                                            \
   T(DataCloneDeserializationError, "Unable to deserialize cloned data.")       \
   T(DataCloneDeserializationVersionError,                                      \
     "Unable to deserialize cloned data due to invalid or unsupported "         \
     "version.")
 
 class MessageTemplate {
  public:
-  enum Template {
+  enum Template : unsigned {
 #define TEMPLATE(NAME, STRING) k##NAME,
     MESSAGE_TEMPLATES(TEMPLATE)
 #undef TEMPLATE
         kLastMessage
   };
 
   static const char* TemplateString(int template_index);
 
   static MaybeHandle<String> FormatMessage(int template_index,
                                            Handle<String> arg0,
@@ skipping 24 matching lines @@
   static Handle<String> GetMessage(Isolate* isolate, Handle<Object> data);
   static std::unique_ptr<char[]> GetLocalizedMessage(Isolate* isolate,
                                                      Handle<Object> data);
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MESSAGES_H_