Add const to TYPELIB pointers

In many places in the code, a const qualifier can be added to TYPELIB pointers:

Members:

• Field_enum::typelib
• Column_definition_attributes::interval
• Create_field::save_interval
• Item_type_holder::enum_set_typelib
• Type_holder::m_typelib

Return values in methods returning a TYPELIB pointer:

• Field::get_typelib()
• Type_all_attributes::get_typelib()

Related method and function parameters getting a TYPELIB pointer should also be fixed accordingly.