This is useful for debugging drop-tracking; previously, you had to recompile
rustc from source and manually add a call to `write_graph_to_file`. This
makes the option more discoverable and configurable at runtime.

I also took the liberty of making the labels for the CFG nodes much easier to read:
previously, they looked like `id(2), local_id: 48`, now they look like
```
expr from_config (hir_id=HirId { owner: DefId(0:10 ~ default_struct_update[79f9]::foo), local_id: 2})
```

Some of these are a little questionable because the output is so much longer, but I would really love to keep the bit that adds the pretty-printed expression to the generated CFG .dot file.

Before:
```
DEBUG rustc_typeck::check::generator_interior::drop_ranges::record_consumed_borrow consume PlaceWithHirId { hir_id: HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, place: Place { base_ty: impl std::future::Future<Output = ()>, base: Rvalue, projections: [] } }; diag_expr_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, using parent expr HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 49 }
```

After:
```
DEBUG rustc_typeck::check::generator_interior::drop_ranges::record_consumed_borrow consume PlaceWithHirId { hir_id: HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, place: Place { base_ty: impl std::future::Future<Output = ()>, base: Rvalue, projections: [] } }; diag_expr_id=expr from_config(Config {
        nickname: None,
        ..Default::default()
    }) (hir_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }), using parent expr
    .await (hir_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 49 })
```