mark vec::IntoIter pointers as `!nonnull`
This applies the same NonNull optimizations to `vec::IntoIter` as #113344 did for `slice::Iter`
[Godbolt](https://rust.godbolt.org/z/n1cTea718) showing the test IR on current nightly, note the absence of `!nonnull` on the loads.
r? `@scottmcm`