cql parser: fix conversion from uninitalized<T> to optional<T> with gcc 9

We use uninitialized<T> (wrapping an optional<T>) to adjust to the
parser's way of laying out the code, but this fails with gcc 9
(presumably for the correct reasons) when converting from
uninitialized<T> back to optional<T>. Add a conversion operator
to make it build.

cql3/Cql.g

@@ -125,6 +125,7 @@ struct uninitialized {
     uninitialized& operator=(uninitialized&&) = default;
     operator const T&() const & { return check(), *_val; }
     operator T&&() && { return check(), std::move(*_val); }
+    operator std::optional<T>&&() && { return check(), std::move(_val); }
     void check() const { if (!_val) { throw std::runtime_error("not intitialized"); } }
 };