[clang] [Clang] Reland "Diagnose UB and emit error when identifier has both internal and external linkage" (PR #193567)

[Aditya Medhane via cfe-commits]

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+// RUN: %clang_cc1 -verify -fsyntax-only %s
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flash1729 wrote:

I kept them separate cause these test cases include a few other shadowing edge cases too, beyond the ones in the paper, but looking back it'd be perfectly fine to fold all of those into n3410.c

Let me know if that's preferred and I'll move them over.

https://github.com/llvm/llvm-project/pull/193567