Rollup merge of #130727 - compiler-errors:objects, r=RalfJung

Check vtable projections for validity in miri

Currently, miri does not catch when we transmute `dyn Trait<Assoc = A>` to `dyn Trait<Assoc = B>`. This PR implements such a check, and fixes https://github.com/rust-lang/miri/issues/3905.

To do this, we modify `GlobalAlloc::VTable` to contain the *whole* list of `PolyExistentialPredicate`, and then modify `check_vtable_for_type` to validate the `PolyExistentialProjection`s of the vtable, along with the principal trait that was already being validated.

cc ``@RalfJung``
r? ``@lcnr`` or types

I also tweaked the diagnostics a bit.

---

**Open question:** We don't validate the auto traits. You can transmute `dyn Foo` into `dyn Foo + Send`. Should we check that? We currently have a test that *exercises* this as not being UB:

https://github.com/rust-lang/rust/blob/6c6d210089e4589afee37271862b9f88ba1d7755/src/tools/miri/tests/pass/dyn-upcast.rs#L14-L20

I'm not actually sure if we ever decided that's actually UB or not 🤔

We could perhaps still check that the underlying type of the object (i.e. the concrete type that was unsized) implements the auto traits, to catch UB like:

```rust
fn main() {
    let x: &dyn Trait = &std::ptr::null_mut::<()>();
    let _: &(dyn Trait + Send) = std::mem::transmute(x);
    //~^ this vtable is not allocated for a type that is `Send`!
}
```

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