[C++20] [Modules] Allow export redeclarations within language linkage

Close https://github.com/llvm/llvm-project/issues/98583

Currently, clang will reject the following code:

```
export module mod;
extern "C++" void func();
export extern "C++" {
    void func();
}
```

while both MSVC and GCC accept it. Although clang's behavior matches the
current wording, from the discussion, the consensus is that we should
accept the above example from the intention. Since the intention to not
allow export redeclaration which is not exported is to make the linkage
clear. But it doesn't matter with the declarations within global module.
2 files changed
tree: 78b49d295410adbba1292f790335b131debbf442
  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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