Fixes#15012.
```python
def f():
# panics when the code can't find the loop variable
values = [1, 2, 3]
result = []
for i in values:
result.append(i + 1)
del i
```
I'm not sure exactly why this test case panics, but I suspect the `del
i` removes the binding from the semantic model's symbols.
I changed the code to search for the correct binding by directly
iterating through the bindings. Since we know exactly which binding we
want, this should find the loop variable without any complications.
Fixes#14969.
The issue was that this line:
```rust
let from_assign_to_loop = TextRange::new(binding_stmt.end(), for_stmt.start());
```
was not safe if the binding was after the target. The only way (at least
that I can think of) this can happen is if they are in different scopes,
so it now checks for that before checking if there are usages between
the two.
