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

[Aaron Ballman via cfe-commits]

================
@@ -1,45 +1,106 @@
-// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c2y -std=c2y -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c89-23 -std=c23 -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c89-23 -std=c17 -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c89-23 -std=c11 -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c89-23 -std=c99 -Wall -pedantic -Wno-unused %s
+// RUN: %clang_cc1 -verify=c89-23 -std=c89 -Wall -pedantic -Wno-unused -Wno-comment %s
 
-/* WG14 N3410: No
+/* WG14 N3410: Clang 23
----------------
AaronBallman wrote:

```suggestion
/* WG14 N3410: Clang 24
```

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