Rollup merge of #107304 - Nilstrieb:ᐸTy as PartialEqᐳ：：eq because what else are we...

Use `can_eq` to compare types for default assoc type error

This correctly handles inference variables like `{integer}`. I had to move all of this `note_and_explain` code to `rustc_infer`, it made no sense for it to be in `rustc_middle` anyways.

The commits are reviewed separately.

Fixes #106968