Remove noexcept on DoFAccessor move constructor and assignment operator

[simonsticko]

I can't can't compile master with clang 6.0.0 without removing the noexcept keyword on DoFAccessor:

In file included from /home/simon/workspace/dealiiCloseToMaster/source/numerics/data_out.cc:18:                                                                          
/home/simon/workspace/dealiiCloseToMaster/include/deal.II/dofs/dof_accessor.h:280:3: error: exception specification of explicitly defaulted move constructor does not    
      match the calculated one                                                                                                                                           
  DoFAccessor(DoFAccessor<structdim, dim, spacedim, level_dof_access>                                                                                                    
  ^                                                                                                                                                                      
/home/simon/workspace/dealiiCloseToMaster/include/deal.II/dofs/dof_accessor.h:1367:32: note: in instantiation of template class 'dealii::DoFAccessor<1, 1, 1, false>'    
      requested here                                                                                                                                                     
class DoFCellAccessor : public DoFAccessor<dimension_,                                                                                                                   
                               ^                                                                                                                                         
/home/simon/workspace/dealiiCloseToMaster/include/deal.II/grid/tria_iterator.h:277:25: note: in instantiation of template class 'dealii::DoFCellAccessor<1, 1, false>'   
      requested here                                                                                                                                                     
    const Triangulation<Accessor::dimension, Accessor::space_dimension> *parent,                                                                                         
                        ^                                                                                                                                                
/home/simon/workspace/dealiiCloseToMaster/include/deal.II/grid/tria_iterator.h:577:29: note: in instantiation of template class                                          
      'dealii::TriaRawIterator<dealii::DoFCellAccessor<1, 1, false> >' requested here                                                                                    
class TriaIterator : public TriaRawIterator<Accessor>                                                                                                                    
                            ^                                                                                                                                            
/home/simon/workspace/dealiiCloseToMaster/source/numerics/data_out.cc:233:21: note: in instantiation of template class 'dealii::TriaIterator<dealii::DoFCellAccessor<1,  
      1, false> >' requested here
                    dh_cell(&cell_and_index->first->get_triangulation(),
                    ^
/home/simon/workspace/dealiiCloseToMaster/build/source/numerics/data_out.inst:127:16: note: in instantiation of member function 'dealii::DataOut<1, 1>::build_one_patch'
      requested here
template class DataOut< 1 >;

Same problem as #12313.

[masterleinad]

/rebuild

[simonsticko]

clang-tidy complained so I had to spam all lines with \\ NO LINT. It doesn't look very nice combined with clang-format, but I don't know what else to do here:

  DoFAccessor<structdim, dim, spacedim, level_dof_access> &       // NOLINT
  operator=(                                                      // NOLINT
    DoFAccessor<structdim, dim, spacedim, level_dof_access> &&) = // NOLINT
    default;                                                      // NOLINT

[masterleinad]

It's a pretty unfortunate situation we are in with clang-tidy also wanting to enforce noexecpt where the compiler should come up with the correct noexcept specifier and older clang versions complaining if they don't think it's correct. 🙁
I think it's fine to comment multiple lines here even if it doesn't look nice.

[bangerth]

Could we teach clang-tidy to just not care about the noexcept?

[masterleinad]

Could we teach clang-tidy to just not care about the noexcept?

We could disable checking that move constructors and move assignment operators should be noexcept but that would also apply to non-defaulted user-provided ones.