If you're doing development in Solr trunk and want to adjust the QueryParser, take a look at the JavaCC grammar file at lucene/solr/core/src/java/org/apache/solr/parser/QueryParser.jj. This isn't a tutorial about JavaCC - there are plenty of those out there.
Once your changes are complete, you'll need to generate the underlying classes again. ant builds from lucene/ or lucene/solr/ don't accomplish this. So to do this, run 'ant javacc' from lucene/solr/core/.
That's it. Continue on with your normal build patterns - I often run 'ant example' from lucene/solr/ when debugging locally. It might make sense for parent builds to always run the javacc target in lucene/solr/core/ but I assume things are set up this way for a reason.