Fix printing negative signed literal in print_literal

[koshlo]

Fix for issue #18091

The problem seems to be that ast_util::int_ty_to_string takes unsigned number, and no one adds - to result string. I've fixed it by putting - before result string using format!.

I've also added test_signed_int_to_string() to check if implementation is valid.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

[alexcrichton]

Thanks! Just to confirm, if you run this through --pretty expanded, it still has the negative sign, right?

fn main() {
    let a = -42i;
}

[koshlo]

Yes, after running this code through --pretty expanded it does contain negative sign.

[alexcrichton]

Thanks @koshlo!