fixed c-api for undo

Author: kr-2003

lib/Interpreter/CXCppInterOp.cpp

@@ -306,7 +306,10 @@ TInterp_t clang_Interpreter_takeInterpreterAsPtr(CXInterpreter I) {
 
 enum CXErrorCode clang_Interpreter_undo(CXInterpreter I, unsigned int N) {
 #ifdef CPPINTEROP_USE_CLING
-  return CXError_Failure;
+  auto* interp = getInterpreter(I);
+  cling::Interpreter::PushTransactionRAII RAII(interp);
+  interp->unload(N);
+  return CXError_Success;
 #else
   return getInterpreter(I)->Undo(N) ? CXError_Failure : CXError_Success;
 #endif // CPPINTEROP_USE_CLING