Bug#15936817: Table left join view, unmatched rows problem where view has an if
Bug#15967464: View evaluation incorrect when joining to view with literal
Bug#20708288: Literal selected from derived table mentioned in outer join ..null
Bug#20841369: Left join to view with <> test causing too many results

This problem may occur for views and derived tables that are on the
inner side of an outer join and from which we select non-nullable
expressions such as literals. If the current row being produced by
the executor contains null-extended values from the inner table(s),
the non-nullable expressions are not presented with NULL values as
they should.

When field translations are created for a derived table or view
reference, we simply set item->maybe_null= true for all fields from
the inner side of an outer join. But for some field types, such as
literals, this does not make sense, because evaluating such fields
can never produce a NULL value.

The solution to the problem is to extend the functionality of class
Item_direct_view_ref. All columns selected from a view are wrapped
in such Item objects. We enhance this class by making nullability
depending on being on the inner side of an outer join, and being
able to take NULL value from a null-extended row.

Tests are added for these Item types, which all can be non-nullable
unless they are selected from the inner side of an outer join.

- literals
- IS [NOT] NULL
- IS [NOT] TRUE
- IS [NOT] FALSE
- IS [NOT] UNKNOWN
- IF
- CASE
- COALESCE
- NULLIF
- IFNULL
- IF

A bug in nullability detection for CASE expressions was discovered
during debugging and fixed.

An unused constructor for class Item_direct_view_ref was deleted.