[red-knot] Symbol API improvements, part 2 (#14276)
## Summary
Apart from one small functional change, this is mostly a refactoring of
the `Symbol` API:
- Rename `as_type` to the more explicit `ignore_possibly_unbound`, no
functional change
- Remove `unwrap_or_unknown` in favor of the more explicit
`.ignore_possibly_unbound().unwrap_or(Type::Unknown)`, no functional
change
- Consistently call it "possibly unbound" (not "may be unbound")
- Rename `replace_unbound_with` to `or_fall_back_to` and properly handle
boundness of the fall back. This is the only functional change (did not
have any impact on existing tests).
relates to: #14022
## Test Plan
New unit tests for `Symbol::or_fall_back_to`
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
