MatrixFree: Add instantiation for ConstraintValues::insert_entries

[kronbichler]

This should fix the build error observed on gcc-6 here: https://cdash.dealii.43-1.org/viewBuildError.php?buildid=1627

Note that I removed the instantiation ConstraintValues<float>, because we never use that one in the code, all uses are here:

dealii/include/deal.II/matrix_free/matrix_free.templates.h

Lines 1813 to 1832 in 7d391a6

	internal::MatrixFreeFunctions::ConstraintValues<double> constraint_values;
	const std::vector<bool> is_fe_dg = internal::compute_dof_info(
	constraint,
	locally_owned_dofs,
	dof_handlers,
	shape_info_dummy,
	cell_level_index_end_local,
	additional_data.mg_level,
	additional_data.hold_all_faces_to_owned_cells,
	additional_data.cell_vectorization_category,
	additional_data.cell_vectorization_categories_strict,
	do_face_integrals,
	additional_data.mapping_update_flags_inner_faces != update_default,
	additional_data.overlap_communication_computation,
	task_info,
	cell_level_index,
	dof_info,
	face_setup,
	constraint_values,
	additional_data.communicator_sm != MPI_COMM_SELF);

and in the functions consuming this data structure.

[kronbichler]

Relates to #13703.