queryparser/lemon.c: Sync change from upstream: "Modify the
%nonassoc directive in lemon so that it generates a run-time error
rather than a parsing conflict. This changes is due to a bug report
on the mailing list. SQLite does not use the %nonassoc directive in
its grammar so this change does not affect SQLite."  Generated code
is identical.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18338 6b2f1b92-a4f3-0310-943f-de3cbe41748b