[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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cor3ntin wrote:

@elhewaty Thanks. LGTM. I'll wait a few days for @AaronBallman to look it over before accepting but it looks like a great fix, thanks

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