[unused_record_fields] Don't count record construction as usage

[elbrujohalcon]

Currently unused_record_fields counts fields in record constructions (e.g. MyRec = #my_rec{field = value} as used (i.e. it doesn't report them) since it can't easily distinguish them from patttern-matching (e.g. #my_rec{field = value} = MyRec).
But assigning a value to a field should not really count as usage.