message: make kythe output and scope resolver const
diff: @@ -271,7 +271,7 @@ class KytheFactsExtractor {
  const VerilogSourceFile* const source_;
  // Output for produced Kythe facts. Not owned.
  - KytheOutput* facts_output_ = nullptr;
  + KytheOutput* const facts_output_;
  // Keeps track of VNames of ancestors as the visitor traverses the facts
  // tree.
  @@ -279,7 +279,7 @@ class KytheFactsExtractor {
  // Keeps track and saves the explored scopes with a <key, value> and maps
  // every signature to its scope.
  - ScopeResolver* scope_resolver_ = nullptr;
  + ScopeResolver* const scope_resolver_;
  // Location signature backing store.
  // Inside signatures, we record locations as string_views,
  