Use `impl`'s generics when suggesting fix on bad `impl Copy`
See the UI test for a more complicated example, but we weren't correctly suggesting to add bounds given a manual `impl` whose generics didn't match the struct generics.
```rust
#[derive(Clone)]
struct Wrapper<T>(T);
impl<S> Copy for Wrapper<S> {}
```
Coincidentally this fix didn't cause any regressions for `derive(Copy)` impls, I think because those use the same spans in the impl generics as the struct generics, so the machinery still applies the same change.
2c14bc3b55