[clang] [clang] Avoid invalid iterator in `MergeDefinitionData` (PR #172559)

[Jonas Hahnfeld via cfe-commits]

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@@ -2107,8 +2107,9 @@ void ASTDeclMerger::MergeDefinitionData(
     auto *Def = DD.Definition;
     DD = std::move(MergeDD);
     DD.Definition = Def;
-    for (auto *D : Def->redecls())
-      cast<CXXRecordDecl>(D)->DefinitionData = ⅅ
+    for (auto *R = Reader.getMostRecentExistingDecl(Def); R;
+         R = R->getPreviousDecl())
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hahnjo wrote:

I guess only @alexfh can give us the final confirmation that all issues are gone, but since this change avoids the iterator it cannot become invalid anymore and the test added in 5ee6cff90ba5d8e08066eeeef0c27aa0b6f24d2c continues to pass.

https://github.com/llvm/llvm-project/pull/172559