Auto merge of #11362 - y21:superfluous_impl, r=xFrednet

new lint: `implied_bounds_in_impls`

Closes #10849

A new lint that looks for explicitly specified bounds that are already implied by other bounds in `impl Trait` return position types.
One example, as shown in the linked example, would be
```rs
fn foo<T>(x: T) -> impl Deref<Target = T> + DerefMut<Target = T> {
    Box::new(x)
}
```
`DerefMut<Target = T>` requires  `Deref<Target = T>` to be wellformed, so specifying `Deref` there is unnecessary.

This currently "ignores" (i.e., does not lint at all) transitive supertrait bounds (e.g. `trait A {} trait B: A {} trait C: B {}`, then having an `impl A + C` type), because that seems a bit more difficult and I think this isn't technically a blocker. It leads to FNs, but shouldn't bring any FPs

changelog: new lint [`implied_bounds_in_impls`]

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