Show interior mutability chain in mutable_key_type

[y21]

Fixes #10619

Just ran into this myself and I definitely agree it's not very nice to have to manually go through all the types involved to figure out why this happens and to evaluate if this is really a problem (knowing if the field of a struct is something that a hash impl relies on), so this changes the lint to emit notes for each step involved.

changelog: none

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[samueltardieu]

Since type names can be quite long sometimes, wouldn't it be best to use … instead of ... here? In monospace font the difference would be two characters wide:

... because it contains `UnsafeCell<usize>`, which has interior mutability
… because it contains `UnsafeCell<usize>`, which has interior mutability

[y21]

I went with the three dots because I think it's much more common in diagnostics (in fact, I think I've never actually seen a non-ASCII character like that in a diagnostic - rust-lang/rust#126597 adds unicode block drawing but it requires an extra --error-format=human-unicode).

Some rustc examples
Some cargo examples

[samueltardieu]

There is a "…" in a diagnostic issued by clippy_lints/src/methods/or_then_unwrap.rs:

clippy_lints/src/methods/or_then_unwrap.rs
25:        title = "found `.or(Some(…)).unwrap()`";
32:        title = "found `.or(Ok(…)).unwrap()`";