Auto merge of #117878 - gavinleroy:proper-depth-check, r=lcnr

Fix depth check in ProofTreeVisitor.

The hack to cutoff overflows and cycles in the new trait solver was incorrect. We want to inspect everything with depth [0..10].

This fix exposed a previously unseen bug, which caused the compiler to ICE when invoking `trait_ref` on a non-assoc type projection. I simply added the guard in the `AmbiguityCausesVisitor`, and updated the expected output for the `auto-trait-coherence` test which now includes the extra note:
```text
   |
   = note: upstream crates may add a new impl of trait `std::marker::Send` for type `OpaqueType` in future versions
```

r? `@lcnr`

Author: bors