parens_expression
token = parens_expression(token, &expr->unop, "in expression");
struct token *parens_expression(struct token *token, struct expression **expr, const char *where);
token = parens_expression(token, &expr, "in attribute");
token = parens_expression(token, &expr, "in attribute");
token = parens_expression(token, &expr, "in attribute");
token = parens_expression(token, &expr, "after _Alignas");
token = parens_expression(token, &op->expr, "in asm parameter");
token = parens_expression(token->next, &expr, "after 'while'");
token = parens_expression(token, &expr, "after 'do-while'");
token = parens_expression(token->next, &stmt->if_conditional, "after if");
token = parens_expression(token->next, &stmt->switch_expression, "after 'switch'");