Rollup merge of #120952 - saethlin:vec-into-iter, r=the8472

Don't use mem::zeroed in vec::IntoIter

`mem::zeroed` is not a trivial function. Maybe it was once, but now it involves multiple locals, copies, and an intrinsic that gets monomorphized into a call to `panic_nounwind` for iterators of types like `Vec<&T>`. Of course all that complexity is trivially optimized out, but generating a bunch of IR where we don't need to just so we can optimize it away later is silly.

Author: matthiaskrgr