Detect non-empty loops

Some loops are clearly executed at least once because they iterate over a non-empty constant. (Or a non-empty typed tuple.)
```python
for x in "a":
y=123
del y
```

```
$ pyright /tmp/tr.py
/tmp/tr.py
/tmp/tr.py:3:5 - error: "y" is possibly unbound (reportPossiblyUnboundVariable)
```
No, it's not.

Start by reproducing the issue with the Python snippet in the report and trace Pyright's analysis of the non-empty loop and the later `del y`. Done means Pyright no longer reports `y` as possibly unbound when the loop iterates over a known non-empty constant or typed tuple.

Written by the indexing model from the issue text.