delegation: support proper interaction of user-specified args and impl Traits

[aerooneqq]

This PR supports usages of user-specified args with impl Traits. When there are user-specified args in child we still need to generate synthetic generic params and use them during signature inheritance:

fn foo<T, const N: usize>(f: impl FnOnce()) {}

reuse foo::<String, 123> as bar;

//desugaring
fn bar<TSynth: impl FnOnce()>(f: _) {
    foo::<String, 123>(f)
}

When inheriting predicates we process impl Trait ones, so we need generic params to instantiate them. Other approach may involve not generating synthetic generic params and try to filter out those predicates, but fairly generating synthetic params seems more consistent?.

Fixes #154780, part of #118212.

r? @petrochenkov

[rustbot]

HIR ty lowering was modified

cc @fmease

[petrochenkov]

r=me after squashing commits.
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[petrochenkov]

@bors delegate+

[rust-bors]

✌️ @aerooneqq, you can now approve this pull request!

If @petrochenkov told you to "r=me" after making some further change, then please make that change and post @bors r=petrochenkov.