Auto merge of #119338 - compiler-errors:upcast-plus-autos, r=lcnr

Consider principal trait ref's auto-trait super-traits in dyn upcasting

Given traits like:

```rust
trait Subtrait: Supertrait + Send {}
trait Supertrait {}
```

We should be able to upcast `dyn Subtrait` to `dyn Supertrait + Send`. This is not currently possible, because when upcasting, we look at the list of auto traits in the object type (`dyn Subtrait`, which has no auto traits in its bounds) and compare them to the target's auto traits (`dyn Supertrait + Send`, which has `Send` in its bound).

Since the target has auto traits that are not present in the source, the upcasting fails. This is overly restrictive, since `dyn Subtrait` will always implement `Send` via its built-in object impl. I propose to loosen this restriction here.

r? types

---

### ~~Aside: Fix this in astconv instead?~~

### edit: This causes too many failures. See rust-lang/rust#119825 (comment)

We may also fix this by by automatically elaborating all auto-trait supertraits during `AstConv::conv_object_ty_poly_trait_ref`. That is, we can make it so that `dyn Subtrait` is elaborated into the same type of `dyn Subtrait + Send`.

I'm open to considering this solution instead, but it would break coherence in the following example:

```rust
trait Foo: Send {}

trait Bar {}
impl Bar for dyn Foo {}
impl Bar for dyn Foo + Send {}
//~^ This would begin to be an overlapping impl.
```

Author: bors