instantiating impl generic args with `'empty` is unsound

originally found by @carlini as "bug 5" in https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/Reporting.20a.20collection.20of.20soundness.20bugs.20in.20next-solver/near/617737394

use std::fmt::Display;
trait ToObj<'a> {
    fn to_obj(self) -> Box<dyn Display + 'a>;
}
impl<'a, T: Display + 'a> ToObj<'a> for T {
    fn to_obj(self) -> Box<dyn Display + 'a> {
        Box::new(self)
    }
}

trait Super {
    fn yeet(&self);
}
impl<'a> Super for ()
where
    for<'b> &'b str: ToObj<'a>,
{
    fn yeet(&self) {
        let dangling = String::from("hello").as_str().to_obj();
        println!("{dangling}");
    }
}

trait Trait: Super {}
impl Trait for () {}

fn main() {
    let _ = (&() as &dyn Trait).yeet();
}

This incorrectly compiles with both the old and the new solver.

The problem is that lexical region resolution supports 'empty and considers for<'b> 'b: 'a to hold if 'a is 'empty. leak_check and MIR borrowck do not have the notion of 'empty anymore.

Start by reproducing the supplied Rust example and tracing lexical region resolution for the 'empty region. Compare that behavior with leak_check and MIR borrowck, which the issue says lack the same notion. Done means the unsound example is rejected by both the old and new solver paths.