[ty] Compact retained use-def bindings (#25682)
## Summary
`UseDefMap` currently retains every interned binding state as a
`SmallVec`, including builder-only capacity and an optional unbound
constraint that are no longer needed after semantic indexing finishes.
This change packs the shadowing policy into the scope-local definition
ID and stores retained binding states as ranges into one contiguous
`LiveBinding` array. Binding construction and interning behavior remain
unchanged, while each `LiveBinding` shrinks from 16 to 12 bytes and
retained states no longer carry per-vector allocation metadata.
On a large codebase, this reduced retained memory by 2.96% without a
wall-time regression.
