[C++20] [Modules] Identify the tag redecl through Using (#221147)

Close https://github.com/llvm/llvm-project/issues/96423

We've already have some logics to merge the redecls. But if the previous
decls are found through using decl, we can't merge them right now. As
the reproducer shows. The using decl is common in modules as a common
method to wrap headers into a module.

The fix is simple too. If the existing check fails, we'll check if the
found one is a using decl, if yes, we will reuse the existing check with
the target decl for using decl.
4 files changed
tree: 39751aa97d3852f907ff98d8c91392a4d0c5cef1
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
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