code: public void syncFilename(String fileName) {
      if (emitLineDirectives) {
        if (!fileName.equals(currentFile)) {
          currentLine = BEGINNING_OF_FILE;
          // C11 spec. (6.10.4) requires a line number between 1 and 2147483647.
          buffer.append(UnicodeUtils.format("\n#line 1 \"%s\"\n", fileName));
        }
      }
      currentFile = fileName;
    }
docs: Emits a #line directive setting the given filename.
  @param fileName the filename to sync