Add new lint stacked_if_match

[promptkp]

fixes #12483

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changelog: new [stacked_if_match] lint against if if and match match

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Alexendoo (or someone else) some time within the next two weeks.

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[bors]

☔ The latest upstream changes (presumably #11476) made this pull request unmergeable. Please resolve the merge conflicts.

[alex-semenyuk]

In theory cases with match if and if match should work? I would be nice to have examples what happens in this cases

[promptkp]

Hi @alex-semenyuk, thank you for the comment. To prevent overlinting, stacked_if_match only lints against if if and match match. I'll add additional test cases for match if and if match.

Note: this is my first contribution to Clippy, so if there's a faster/cleaner way to implement this, please let me know!

[bors]

☔ The latest upstream changes (presumably #13247) made this pull request unmergeable. Please resolve the merge conflicts.