Consider using the Open Fortran Parser
The Open Fortran Parser is based on ANTLR, just like the current Fortran parser in LFortran. Maybe it can serve as a foundation to build LFortran upon, since it already implements the Fortran 2008 standard. Or at least it could be used temporarily while LFortran catches up with it.
I hope it can help save time and effort, which are better spent implementing the cool features that set LFortran apart from other compilers.