[ty] Use ThinVec for sparse kwargs bindings (#25457)

## Summary

`multi_bindings_by_use` is only populated for kwargs expressions (like
`f(**options)`), so it is empty or very small for most scopes. This PR
keeps the mutable hash map while building the use-def map, then converts
it into a sorted `ThinVec` for storage, which is much smaller
(especially when empty). Lookups use binary search by `ScopedUseId`.