Rollup merge of #135900 - compiler-errors:derive-wf, r=lcnr

Manually walk into WF obligations in `BestObligation` proof tree visitor

When we encounter a `WellFormed` obligation in the `BestObligation` proof tree visitor, ignore the proof tree and call `wf::unnormalized_obligations` to derive well-formed obligations with the correct cause codes. This is to avoid having to replicate the somewhat delicate logic that `wf.rs` does to set up its obligation causes... Don't see a better way to do this.

vibes?? r? lcnr

Author: matthiaskrgr