NLL: missing "consider using a let binding" message

[nikomatsakis]

(NB. This is assuming my in-progress branch)

If you run issue-36082.rs in NLL mode, you get:

> rustc --stage1 issue-36082.rs  -Zborrowck=mir
error[E0597]: borrowed value does not live long enough
  --> issue-36082.rs:21:19
   |
21 |     let val: &_ = x.borrow().0;
   |                   ^^^^^^^^^^  - temporary value only lives until here
   |                   |
   |                   temporary value does not live long enough
...
30 |     println!("{}", val);
   |                    --- borrow later used here

There is no suggestion about using a let binding to increase the lifetime of the temporary, unlike AST borrowck:

> rustc --stage1 issue-36082.rs
error[E0597]: borrowed value does not live long enough
  --> issue-36082.rs:21:19
   |
21 |     let val: &_ = x.borrow().0;
   |                   ^^^^^^^^^^  - temporary value dropped here while still borrowed
   |                   |
   |                   temporary value does not live long enough
...
31 | }
   | - temporary value needs to live until here
   |
   = note: consider using a `let` binding to increase its lifetime

That said, this seems like a case where the AST borrowck message could also be improved. I doubt many people understand it very well.

[nikomatsakis]

I'm lowering this to the Release milestone, as it seems like a relatively minor nit. Still good to see it fixed though!