NLL should respect lifetimes in as expressions

[nikomatsakis]

Spawned off from #47184. The following test case compiles, but it should not:

#![allow(warnings)]
#![feature(nll)]

fn main() {
    let x = 22_u32;
    let y: &u32 = (&x) as &'static u32;
}

The problem is that the "cast" expression is not preserving the full type that the user gave (&'static u32).

This is very similar to #54331

[nikomatsakis]

In fact, I would handle this by just wrapping casts in one of the ExprKind::Type nodes that are added in #54331

[nikomatsakis]

I'm assigning this to @KiChjang since it is a good follow-up to the work they've been doing