[clang] [clang] emit an error when the same identifier appears with both internal and external linkage in a translation unit (PR #78064)

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elhewaty wrote:

Hello @cor3ntin, can you please suggest `clang` issues that maybe suitable for a beginner, or if you have a TO-DO list I will be happy to work on it.
Thanks

https://github.com/llvm/llvm-project/pull/78064