Using a `ProofTreeVisitor` when computing `obligations_for_self_ty`

[compiler-errors]

Due to pending obligations in the new fulfillment context recording only the root obligations and not the leaf obligations that have stalled, this code fails to compile:

trait Foo {}
fn needs_foo<T>(_: T) where Wrap<T>: Foo {}

struct Wrap<T>(T);
impl<T> Foo for Wrap<T> where T: Fn(i32) {}

fn main() {
    needs_foo(|x| {
        x.to_string();
    });
}

Recording this here so I can add a test in the UI test suite for it, since if we ever experiment with removing pending_obligations, this will likely go from pass -> fail.

[lcnr]

With rust-lang/rust#122385 the new solver now uses a ProofTreeVisitor to also look at nested obligations

[lcnr]

we only need to make sure we've documented this somewhere