make `for PAT in ITER_EXPR { ... }` a terminating-scope for ITER_EXPR.

In effect, temporary anonymous values created during the evaluation of
ITER_EXPR no longer not live for the entirety of the block surrounding
the for-loop; instead they only live for the extent of the for-loop
itself, and no longer.

----

There is one case I know of that this breaks, demonstrated to me by
niko (but it is also a corner-case that is useless in practice).  Here
is that case:

```
fn main() {
    let mut foo: Vec<&i8> = Vec::new();
    for i in &[1, 2, 3] { foo.push(i) }
}
```

Note that if you add any code following the for-loop above, or even a
semicolon to the end of it, then the code will stop compiling (i.e.,
it gathers a vector of references but the gathered vector cannot
actually be used.)

(The above code, despite being useless, did occur in one run-pass test
by accident; that test is updated here to accommodate the new
striction.)

----

So, technically this is a:

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