_children_valid_format of OpenMP directives

[LonelyCat124]

As of #1151, OpenMP directives have additional "Clause" children.. At the moment, the specifier in _children_valid_format requires an ordering of these clauses as children, as something like [OMPPrivateClause, OMPDefaultClause]* doesn't specify a maximum of each number of children.

@sergisiso has had a few thoughts on this, e.g.:

I am still not sure what is the nicest solution here. Another option we talked about in to have a ClausesList which is mandatory and always proceed the directive body. And inside having the clauses/ or nothing if all are optional.

however, right now we did not have a solution we were happy with.