[clang] Diagnose redeclaration with different linkage (PR #178900)

[Oliver Hunt via cfe-commits]

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@@ -9210,9 +9210,21 @@ bool Sema::CheckVariableDeclaration(VarDecl *NewVD, LookupResult &Previous) {
     Previous.setShadowed();
 
   if (!Previous.empty()) {
-    MergeVarDecl(NewVD, Previous);
-    return true;
+  if (auto *OldDecl =
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ojhunt wrote:

I think this logic probably belongs in MergeVarDecl itself

https://github.com/llvm/llvm-project/pull/178900