[clang] [Clang] strengthen checks for 'main' function to meet [basic.start.main] p3 requirements (PR #101853)

[via cfe-commits]

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@@ -145,6 +145,8 @@ Improvements to Clang's diagnostics
 
 - -Wdangling-assignment-gsl is enabled by default.
 
+- Clang now diagnoses the use of `main` in an `extern "C"` context as invalid according to [basic.start.main] p3. Fixes #GH101512.
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Sirraide wrote:

```suggestion
- Clang now diagnoses the use of `main` in an `extern` context as invalid according to [basic.start.main] p3. Fixes #GH101512.
```

Actually, my bad, I didn’t realise that `extern "C++" int main()` was also invalid at global scope, so this should in fact just say `extern` .

https://github.com/llvm/llvm-project/pull/101853