- Rename `ScopeIndex` to `ScopeId`, because the order is not meaningful.
This avoids collisions with `SemIR::ScopeIndex`.
- Change `GetScopeFor` to use an `if constexpr` chain rather than
duplicating the numbering logic across multiple functions.
- Simplify `GetNameFor`, avoiding multiple identical overloads.
As requested in review of #3683.