[clang][ASTImporter] support import return with UnaryTransformType (#101517)

This fixes infinite recursion crash on return with UnaryTransformType,
whose underlying type is a SubstTemplateTypeParmType which is
associated with current imported function.

Author: danix800

clang/lib/AST/ASTImporter.cpp

@@ -3640,6 +3640,10 @@ class IsTypeDeclaredInsideVisitor
     return {};
   }
 
+  std::optional<bool> VisitUnaryTransformType(const UnaryTransformType *T) {
+    return CheckType(T->getBaseType());
+  }
+
   std::optional<bool>
   VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T) {
     // The "associated declaration" can be the same as ParentDC.

clang/unittests/AST/ASTImporterTest.cpp

@@ -7598,6 +7598,25 @@ TEST_P(ImportAutoFunctions, ReturnWithSubstNonTypeTemplateParmExpr) {
   EXPECT_TRUE(ToBar);
 }
 
+TEST_P(ImportAutoFunctions, ReturnWithUnaryTransformType) {
+  const char *Code =
+      R"(
+        enum E { E1 };
+
+        template<typename T>
+        auto foo(T v) { return static_cast<__underlying_type(T)>(v); }
+
+        bool bar() { return foo(E1); }
+      )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX17);
+
+  auto *FromBar = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("bar")));
+
+  auto *ToBar = Import(FromBar, Lang_CXX17);
+  EXPECT_TRUE(ToBar);
+}
+
 struct ImportSourceLocations : ASTImporterOptionSpecificTestBase {};
 
 TEST_P(ImportSourceLocations, PreserveFileIDTreeStructure) {