parse: include rule head location when requested

Prior to this commit, when the AST information
were serialized to JSON, rules expressed as

discussed in #5790 included only the location
of their value and not the one of the rule
ref var itself. Now the AST serialized to
JSON includes this information.

Fixes: #5790
Signed-off-by: Gianluca Oldani <oldanigianluca@gmail.com>