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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -ast-dump -std=c++17 %s | FileCheck %s
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AaronBallman wrote:

```suggestion
// RUN: %clang_cc1 -ast-dump %s | FileCheck %s
```
I don't think there's anything which requires C++17 in the test?

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