message: Downgrade attribute "extends" of enum alias elements from required to optional.
diff: @@ -14010,7 +14010,7 @@ void VulkanHppGenerator::readRequireEnumAlias( tinyxml2::XMLElement const *
  std::map<std::string, std::string> const & attributes )
  {
  int line = element->GetLineNum();
  - checkAttributes( line, attributes, { { "alias", {} }, { "extends", {} }, { "name", {} } }, { { "comment", {} } } );
  + checkAttributes( line, attributes, { { "alias", {} }, { "name", {} } }, { { "comment", {} }, { "extends", {} } } );
  checkElements( line, getChildElements( element ), {} );
  std::string alias, bitpos, name, extends, extnumber, offset, value;
  @@ -14030,12 +14030,15 @@ void VulkanHppGenerator::readRequireEnumAlias( tinyxml2::XMLElement const *
  }
  }
  + if ( !extends.empty() )
  + {
  auto enumIt = m_enums.find( extends );
  check( enumIt != m_enums.end(), line, "feature extends unknown enum <" + extends + ">" );
  // add this enum name to the list of aliases
  enumIt->second.addEnumAlias( line, name, alias );
  }
  +}
  void VulkanHppGenerator::readRequires( tinyxml2::XMLElement const * element,
  std::map<std::string, std::string> const & attributes )
  