[clang] [clang-repl] Unlink the withdrawn partial translation unit on Undo (PR #213235)

[Vassil Vassilev via cfe-commits]

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@@ -231,6 +230,9 @@ void IncrementalParser::CleanUpPTU(TranslationUnitDecl *MostRecentTU) {
         !D->getLangOpts().CPlusPlus)
       S.IdResolver.RemoveDecl(ND);
   }
+
+  // Lookup alone is not enough: the redeclaration chain still reaches these.
+  S.getASTContext().withdrawTranslationUnitDecl();
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vgvassilev wrote:

This change should be done local to the interpreter infrastructure. We can disconnect the last PTU from the redecl chain by effectively rebuilding it. You can take a look at how cling does it. 

https://github.com/llvm/llvm-project/pull/213235