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

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Sirraide wrote:

Ah, actually, this is still missing a release note (in `clang/docs/ReleaseNotes.rst`, in the ‘Improvements to Clang's diagnostics’ section)

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