Migrate TargetFolder copy constructor

[xiangzhai]

for example:

// Copy assignment operator.                                                      
  FieldContents &operator=(const FieldContents & other) {                           
    R = other.R;                                                                    
    C = other.C;                                                                    
    Starts = other.Starts;                                                          
#if LLVM_VERSION_CODE < LLVM_VERSION(3, 9)                                          
    Folder = other.Folder;                                                          
#endif                                                                              
    return *this;                                                                   
  }