trait_selection: Preserve eager normalization failures

[Dnreikronos]

Draft experiment for rust-lang#161407, based on its branch so the diff shows the alternative discussed on Zulip.

The nested inherent associated type case fails during eager normalization, but the fallback can accept it. Eager normalization reduces the inner alias to &'b (), then tries to equate Foo<for<'b> fn(&'b ())> with the impl's Foo<fn(&'a ())>. The impl lifetime sits outside the for<'b> binder, so that relation fails the leak check. The fallback retries the original nested aliases. That turns the lifetime equalities into constraints returned by nested goals, which instantiate_and_apply_query_response marks VisibleForLeakCheck::No. The parent leak check misses them and the fallback succeeds. Resolving the resulting inference variable at the end hides the rejected relation.

I changed normalize_with_universes to return a Result carrying the failed obligation. Deep normalization returns that error immediately, and I removed the final resolve_vars_if_possible workaround. The general policy for nested constraints stays as it is.

For the infallible normalize entry point, I kept recovery inside the function for now. It catches the error, puts the failed obligation in the list, then runs ReplaceAliasWithInfer over the original value. The visitor still creates inference variables and obligations for aliases without escaping bound vars. For aliases with escaping bound vars, it leaves the alias in place and no longer creates placeholder obligations. Keeping the original failure means a successful fallback can't silently discard it.

My thinking was to get deep normalization to report the failure while keeping the existing normalize callers working. I think making normalization fallible throughout is the better direction, with HIR typeck reporting the failure and replacing aliases with TyKind::Error. I haven't made those caller changes here. That's the part I'd like to settle before taking this further.

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