[clang] [Clang][ASTImporter] Fix cycle in importing template specialization on auto type with typename (PR #162514)

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@@ -254,6 +256,13 @@ class TypeSourceInfo;
     /// Declaration (from, to) pairs that are known not to be equivalent
     /// (which we have already complained about).
     NonEquivalentDeclSet NonEquivalentDecls;
+    /// A FunctionDecl can have properties that have a reference to the
+    /// function itself and are imported before the function is created. This
+    /// can come for example from auto return type or when template parameters
+    /// are used in the return type or parameters. This member is used to detect
+    /// cyclic import of FunctionDecl objects to avoid infinite recursion.
+    std::unique_ptr<FunctionReturnTypeDeclCycleDetector>
+        FunctionReturnTypeCycleDetector;
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ganenkokb-yandex wrote:

as unique_ptr it requires only forward declaration. You need full class declaration otherwise.

https://github.com/llvm/llvm-project/pull/162514