Make all thir types implement clone
This PR adds `Clone` impl to all of the `Thir<'tcx>` types.
I would like to be able to clone a `Thir` body so that I can make a copy in my rustc driver without breaking further compilation. Without this my driver is forced to run in the `after_expansion` callback and thus doesn't benefit from running all the safety checks that `rustc` usually does, instead i need to do them all myself.
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