Auto merge of #11938 - GuillaumeGomez:unconditional_recursion, r=llogiq

Add new `unconditional_recursion` lint

Currently, rustc `unconditional_recursion` doesn't detect cases like:

```rust
enum Foo {
    A,
    B,
}

impl PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        self == other
    }
}
```

This is because the lint is currently implemented only for one level, and in the above code, `self == other` will then call `impl PartialEq for &T`, escaping from the detection. The fix for it seems to be a bit tricky (I started investigating potential solution to add one extra level of recursion [here](https://github.com/rust-lang/rust/compare/master...GuillaumeGomez:rust:trait-impl-recursion?expand=1) but completely broken at the moment).

I expect that this situation will remain for a while. In the meantime, I think it's acceptable to check it directly into clippy for the time being as a lot of easy cases like this one can be easily checked (next I plan to extend it to cover other traits like `ToString`).

changelog: Add new `unconditional_recursion` lint

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