Fix syntax error false positive on nested alternative patterns (#21104)
## Summary
Fixes #21101 by storing the child visitor's names in the parent visitor.
This makes sure that `visitor.names` on line 1818 isn't empty after we
visit a nested OR pattern.
## Test Plan
New inline test cases derived from the issue,
[playground](https://play.ruff.rs/7b6439ac-ee8f-4593-9a3e-c2aa34a595d0)