Define set type from all elements type (or empty_set type).

def visit_Set(self, node):
        """ Define set type from all elements type (or empty_set type). """
        self.generic_visit(node)
        if node.elts:
            for elt in node.elts:
                self.combine(node, elt, unary_op=self.builder.SetType)
        else:
            self.result[node] = self.builder.NamedType(
                "pythonic::types::empty_set")