message: Add more detail to a check failure seen in some Kythe indexings.
diff: @@ -204,8 +204,13 @@ const SyntaxTreeLeaf& GetSubtreeAsLeaf(const S& symbol,
  E parent_must_be_node_enum,
  size_t child_position) {
  internal::MustBeCSTSymbolOrNode(symbol);
  - return SymbolCastToLeaf(*ABSL_DIE_IF_NULL(
  - GetSubtreeAsSymbol(symbol, parent_must_be_node_enum, child_position)));
  + const Symbol* subtree =
  + GetSubtreeAsSymbol(symbol, parent_must_be_node_enum, child_position);
  + // TODO(ikr): avoid this check-failure.
  + CHECK(subtree != nullptr)
  + << symbol.Kind() << " e:" << parent_must_be_node_enum
  + << " p:" << child_position;
  + return SymbolCastToLeaf(*subtree);
  }
  template <class S, class E>
  