Generalize logic pointing at binding moved into closure

Account not only for `fn` parameters when moving non-`Copy` values into closure, but also for let bindings.

```
error[E0507]: cannot move out of `bar`, a captured variable in an `FnMut` closure
  --> $DIR/borrowck-move-by-capture.rs:9:29
   |
LL |     let bar: Box<_> = Box::new(3);
   |         ---  ------ move occurs because `bar` has type `Box<isize>`, which does not implement the `Copy` trait
   |         |
   |         captured outer variable
LL |     let _g = to_fn_mut(|| {
   |                        -- captured by this `FnMut` closure
LL |         let _h = to_fn_once(move || -> isize { *bar });
   |                             ^^^^^^^^^^^^^^^^   ---- variable moved due to use in closure
   |                             |
   |                             `bar` is moved here
   |
help: consider cloning the value before moving it into the closure
   |
LL ~         let value = bar.clone();
LL ~         let _h = to_fn_once(move || -> isize { value });
   |
```

```
error[E0507]: cannot move out of `y`, a captured variable in an `Fn` closure
  --> $DIR/unboxed-closures-move-upvar-from-non-once-ref-closure.rs:12:9
   |
LL |     let y = vec![format!("World")];
   |         -   ---------------------- move occurs because `y` has type `Vec<String>`, which does not implement the `Copy` trait
   |         |
   |         captured outer variable
LL |     call(|| {
   |          -- captured by this `Fn` closure
LL |         y.into_iter();
   |         ^ ----------- `y` moved due to this method call
   |         |
   |         `y` is moved here
   |
note: `into_iter` takes ownership of the receiver `self`, which moves `y`
  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
help: you can `clone` the value and consume it, but this might not be your desired behavior
   |
LL |         <Vec<String> as Clone>::clone(&y).into_iter();
   |         +++++++++++++++++++++++++++++++ +
help: consider cloning the value if the performance cost is acceptable
   |
LL |         y.clone().into_iter();
   |          ++++++++
```

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