[clang] [clang][Sema] Fix diagnostic for function overloading in extern "C" (PR #106033)

[via cfe-commits]

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@@ -261,6 +261,8 @@ Improvements to Clang's diagnostics
   compilation speed with modules. This warning is disabled by default and it needs
   to be explicitly enabled or by ``-Weverything``.
 
+- Improved diagnostics when trying to overload a function in extern "C". (#GH80235)
----------------
Sirraide wrote:

```suggestion
- Improved diagnostics when trying to overload a function in an ``extern "C"`` context. (#GH80235)
```

https://github.com/llvm/llvm-project/pull/106033