Auto merge of #6130 - Ambroisie:lint-ptr-eq, r=Manishearth

Auto merge of #6130 - Ambroisie:lint-ptr-eq, r=Manishearth

New lint: Recommend using `ptr::eq` when possible

This is based almost entirely on the code available in the previous PR #4596. I merely updated the code to make it compile.

Fixes #3661.

- [ ] I'm not sure about the lint name, but it was the one used in the original PR.
- [X] Added passing UI tests (including committed `.stderr` file)
- [X] `cargo test` passes locally
- [X] Executed `cargo dev update_lints`
- [X] Added lint documentation
- [X] Run `cargo dev fmt`

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changelog: none