Rollup merge of #144988 - amandasystems:more-detailed-region-graph, r=lcnr

Add annotations to the graphviz region graph on region origins

This adds
- `(ex<'e>)` for regions whose origin is existential, with name if one exists,
- `(for<'p>)` for regions whose origin is a placeholder, with name if one exists

For any region whose name we don't know, use `'_`.

This has helped _my_ debugging and it doesn't create too bad clutter, I feel.

The change  ~~is~~was ridiculously small, but I turned it into a separate PR so we can bikeshed the format.

The following snippet:
```rust
struct Co<'a>(&'a ());
struct Contra<'a>(fn(&'a ()));

// `exists<'e> forall<'p> 'p: 'e` -> ERROR
fn p_outlives_e(
    x: for<'e> fn(for<'p> fn(fn(fn(Contra<'e>, Co<'p>)))),
) -> fn(fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
    x
```

Gives this graph:
![new-naming](https://github.com/user-attachments/assets/f2c8f17c-d29b-4d42-9da7-4b8e520e76a6)

Author: Zalathar