Auto merge of #156187 - inq:obligations-self-ty-recompute-sub-root, r=BoxyUwU,lcnr

obligations_for_self_ty: skip irrelevant goals (recompute sub_root from stalled_vars)



Supersedes [#146759](https://github.com/rust-lang/rust/pull/146759). Since the original PR is having conflict now, I've rebased.

This includes 2 commit

1. the original one with rebase
2. above that, I've changed:

```
if stalled_on is Some: ignore sub_roots -> walk stalled_vars -> re-evaluate sub_root 
else: same with this PR
```

Why:
cached sub_roots can be stale if there is a sub-unification merge after stalling ([lcnr proposed on the original PR](https://github.com/rust-lang/rust/pull/146759#issuecomment-4378162413))
so re-evaluating when reads ---> so more conservative

Testing:

* next-solver / traits tests
* we can follow after merging rust-lang/rust#156172 with -Zdisable-fast-paths

Style question:

In my preference, inside filter, I used filter_map - matches, but do you prefer if-let chain?

Original author: [lcnr](https://github.com/lcnr) [#146759](https://github.com/rust-lang/rust/pull/146759)

r? @lcnr 
tree: 3666e732e105ccda22bb120b99e6765119768e20
  1. .github/
  2. compiler/
  3. library/
  4. LICENSES/
  5. src/
  6. tests/
  7. .clang-format
  8. .editorconfig
  9. .git-blame-ignore-revs
  10. .gitattributes
  11. .gitignore
  12. .gitmodules
  13. .ignore
  14. .mailmap
  15. bootstrap.example.toml
  16. Cargo.lock
  17. Cargo.toml
  18. CODE_OF_CONDUCT.md
  19. configure
  20. CONTRIBUTING.md
  21. COPYRIGHT
  22. INSTALL.md
  23. LICENSE-APACHE
  24. license-metadata.json
  25. LICENSE-MIT
  26. package.json
  27. README.md
  28. RELEASES.md
  29. REUSE.toml
  30. rust-bors.toml
  31. rustfmt.toml
  32. triagebot.toml
  33. typos.toml
  34. x
  35. x.ps1
  36. x.py
  37. yarn.lock
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