Problem: Segmentation fault in add_group_and_distinct_keys() when accessing
field of what is assumed to be an Item_field object.

Cause: In case of views, the item added to list by is_indexed_agg_distinct() 
was not of type Item_field, but Item_ref.

Resolution:  Add the real Item_field object, the one referred to by 
Item_ref object, to the list, instead.

mysql-test/r/count_distinct.result:
  Results for test case for Bug#51980.
mysql-test/t/count_distinct.test:
  Test case for Bug#51980.
  Table needs to contain at least two rows to avoid const table optimization.
sql/sql_select.cc:
  Make sure it is the actual Item_field object that is pushed to the out_args
  list of is_indexed_agg_distinct(), and not Item_ref objects.