Auto merge of #64432 - gnzlbg:simplify_truncate, r=alexcrichton

Make the semantics of Vec::truncate(N) consistent with slices.

This commit simplifies the implementation of `Vec::truncate(N)` and
makes its semantics identical to dropping the `[vec.len() - N..]`
sub-slice tail of the vector, which is the same behavior as dropping a
vector containing the same sub-slice.

This changes two unspecified aspects of `Vec::truncate` behavior:

* the drop order, from back-to-front to front-to-back,
* the behavior of `Vec::truncate` on panics: if dropping one element of
  the tail panics, currently, `Vec::truncate` panics, but with this PR all other
  elements are still dropped, and if dropping a second element of the tail
  panics, with this PR, the program aborts.

Programs can trivially observe both changes. For example
([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7bef575b83b06e82b3e3529e4edbcac7)):

```rust
fn main() {
    struct Bomb(usize);
    impl Drop for Bomb {
        fn drop(&mut self) {
            panic!(format!("{}", self.0));
        }
    }
    let mut v = vec![Bomb(0), Bomb(1)];
    std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        v.truncate(0);
    }));
    assert_eq!(v.len(), 1);
    std::mem::forget(v);
}
```

panics printing `1` today and succeeds. With this change, it panics
printing `0` first (due to the drop order change), and then aborts
with a double-panic printing `1`, just like dropping the
`[Bomb(0), Bomb(1)]` slice does, or dropping
`vec![Bomb(0), Bomb(1)]` does.

This needs to go through a crater run.

r? @SimonSapin

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