[red-knot] optimize is_subtype_of for literals (#17394)
## Summary
Allows us to establish that two literals do not have a subtype
relationship with each other, without having to fallback to a typeshed
Instance type, which is comparatively slow.
Improves the performance of the many-string-literals union benchmark by
5x.
## Test Plan
`cargo test -p red_knot_python_semantic` and `cargo bench --bench
red_knot`.
