we should avoid running our special lifetime checks for external functions

[tjhance]

this is a follow-up to #2812 which patches up lifetime-checking for a verus-unsupported feature. This kind of patch ought to be unnecessary if we just avoid running our lifetime-checking entirely.

[tjhance]

also note that the relevant repro:

use vstd::prelude::*;

fn crash(n: &u64) -> impl Sized {
    let x = &raw const n;
    42u64
}

only reproduces the issue because of the opaque type (which is needed to trigger the lifetime check).

(also note that the effort in #2621 will probably fix this issue)