Re-add `Clone`-derive on `Thir` This PR adds back `Clone` for `Thir`. If a tool wants to access a `thir_body` query result in the `Callbacks::after_analysis` hook, it can't do so (I think) without a `Clone` impl on `Thir`, because `check_unsafety` steals the value. With `Clone`, the `thir_body` query provider can be overriden to cache a clone of the `Thir`, circumventing that issue. Specifically, we need it for https://github.com/rust-corpus/qrates, [here](https://github.com/skius/qrates/blob/ca7a2301968a43862f2c04daffed71a9de8c333c/extractor/src/lib.rs#L205). Please let me know if there are issues with this PR/if there's another way to solve the problem at hand