Upstreming various features implemented for ZetaSQL.

• Error and warning thresholds for rule expansion from EBNF desugaring to prevent surprise exponential automota growth.
• Configurable cap on rule size for ordinal refences in semantic actions to enable weening from that feature.
• Some additional bookkeeping in the main parser loop that can give error message generation better content for generating expected followsets.
• #line directives and better action-function names in C++ to improve debugging compile time errors and runtime crashes in semantic action code.
• Change the default C++ semantic action return value mechanism from union to std::variant for better type safety.