diagnostics: point to coroutine body on higher-ranked auto trait errors

[shivendra02467]

Fixes #155880

When encountering a higher-ranked auto trait bound error involving a coroutine or async function,the trait solver previously used the span of the outermost cause (eg. spawn(...) or is_send(...)).

This PR modifies the TraitPlaceholderMismatch formatting logic to walk down the ObligationCauseCode chain.If the obligation originates from a ty::Coroutine or ty::CoroutineWitness, it extracts that specific span, providing a much more accurate underline for the user.

Before:

error: implementation of `Send` is not general enough
  --> src/main.rs:25:5
   |
25 |     is_send(outer())
   |     ^^^^^^^^^^^^^^^^ implementation of `Send` is not general enough

After:

error: implementation of `Send` is not general enough
  --> src/main.rs:13:74
   |
13 |   async fn inner<'a, T: Trait + 'a>(_: T, x: T::Assoc<'a>) -> T::Assoc<'a> {
   |  __________________________________________________________________________^
14 | |     std::future::ready(x).await
15 | | }
   | |_^ implementation of `Send` is not general enough

[rustbot]

r? @nnethercote

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[nnethercote]

Seems plausible but I don't know enough about the underlying issue to review effectively. Let's try someone else:

r? @lcnr

[shivendra02467]

Hi @lcnr, just a gentle ping on this whenever you have a moment! Since you opened the original issue, I'd love to get your thoughts on this approach.