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lib: specialize Clone of array IntoIter This PR adds the `TrivialClone` specialization to the `PolymorphicIter` type, which `array::IntoIter` uses. I also added another coretest for the default case (already existing test catches the specialized one). There are no unwinding concerns since `TrivialClone` means cloning = copying and we never call `Clone::clone`. `array:IntoIter` is being used on a lot of places in both `core` and in user code, and `array::IntoIter` (nor does its users) override `Clone` often and just delegate to the inner type `PolymorphicIter` which does not yet do the `TrivialClone` specialization and is burdened by the iterator and unwind safety shenanigans for all types, so this PR should address some of that. This should in turn slightly reduce both stack usage and remove unwinding prologue/epilogue etc. from such functions and improve our `-Cno-prepopulate-passes` codegen too. perf: https://godbolt.org/z/eTPjj3hP3