Rollup merge of #126730 - oli-obk:opaque_type_diff_next_solver, r=lcnr

Add opaque type corner case test

r? ``@lcnr``

I can't make sense of the new solver tracing logs yet, so I just added the test without explanation.

The old solver does not yet figure out that `Foo == ()` from the where bounds. Unfortunately, even if we make it understand that, it will later try to prove `<X as Trait<'static>>::Out<Foo>: Sized` via the `is_sized_raw` query, which does not take a list of defineable opaque types, causing that check to fail with an ICE.

Thus I'm submitting this test case on its own just to ensure we handle it correctly in the future with any new solver or old solver changes.

Author: matthiaskrgr