Map RPIT duplicated lifetimes back to fn captured lifetimes

Use the [`lifetime_mapping`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.OpaqueTy.html#structfield.lifetime_mapping) to map an RPIT's captured lifetimes back to the early- or late-bound lifetimes from its parent function. We may be going thru several layers of mapping, since opaques can be nested, so we introduce `TyCtxt::map_rpit_lifetime_to_fn_lifetime` to loop through several opaques worth of mapping, and handle turning it into a `ty::Region` as well.

We can then use this instead of the identity substs for RPITs in `check_opaque_meets_bounds` to address #114285.

We can then also use `map_rpit_lifetime_to_fn_lifetime` to properly install bidirectional-outlives predicates for both RPITs and RPITITs. This addresses #114601.

I based this on #114574, but I don't actually know how much of that PR we still need, so some code may be redundant now... 🤷

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Fixes #114597
Fixes #114579
Fixes #114285

Also fixes #114601, since it turns out we had other bugs with RPITITs and their duplicated lifetime params 😅.

Supersedes #114574

r? `@oli-obk`