[ty] Track literal iterable emptiness for reachability (#25222)
## Summary
This extends the static reachability analysis for synchronous `for`
loops from #25220 to tuple, list, set, dictionary, string, and bytes
literals whose emptiness is known syntactically. We record their static
truthiness: a non-empty literal proves that the body executes at least
once, while an empty literal marks the body unreachable and prevents its
definitions from flowing past the loop.
```python
for item in [1]:
pass
reveal_type(item) # Literal[1]
value = 1
for _ in []:
value = "unreachable"
reveal_type(value) # Literal[1]
```
Starred elements and dictionary unpacking remain ambiguous when they are
the only elements, while an explicit element still proves non-emptiness.
Other iterables continue on the existing ambiguous path. Synchronous
literals used in `async for` do not use this shortcut because they do
not satisfy async iteration.
