[Clang][Concepts] Fix the constraint equivalence checking involving parameter packs (#102131)

We established an instantiation scope in order for constraint
equivalence checking to properly map the uninstantiated parameters.

That mechanism mapped even packs to themselves. Consequently, parameter
packs e.g. appearing in a function call, were not expanded. So they
would end up becoming `SubstTemplateTypeParmPackType`s that circularly
depend on the canonical declaration of the function template, which is
not yet determined, hence the spurious error.

No release note as I plan to backport it to 19.

Fixes https://github.com/llvm/llvm-project/issues/101735

---------

Co-authored-by: cor3ntin <corentinjabot@gmail.com>
2 files changed
tree: 7df56a541fd7d0ec8219b4cc0f77dfa486393a71
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
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