Change the sage calss of OpenMP clause

[ouankou]

Currently, IR for the OpenMP clause is SgOmpClause, which is extended from SgLocatedNodeSupport. At the same level as SgLocatedNodeSupport, there is SgExpression. It may be more suitable for the OpenMP clause because, as part of the directive, each portion, in this case, the clause, could be considered as an expression. Like any expression, it's a tree with some child nodes.

To make the change, we first need to update ROSETTA and let SgOmpClause extend from SgExpression. It should be enough, but we should double-check if anything is broken elsewhere. For now, this is not a high priority, we can leave it as-is.