The problem is that the function get_full_info() expects a field item
but is presented with an Item_func_if item. This is due to the new
orthogonal data type aggregation, which handles columns from UNION
and columns from CASE-type functions equally. Earlier, the IF would
never return an ENUM or SET data type, but this is now possible, when
e.g. the arguments are an enum field and a NULL value.

We have fixed the problem by pushing the typelib retrieval to each
individual Item class. A get_typelib() function for each relevant
Item class has been implemented. Unfortunately, since the CASE classes
do not have a common base class, an implementation function is required
for each class.

We have also renamed the get_full_info() function to set_typelib(),
since its sole purpose is to set typelib information for enum/set types.

Tests are added for most relevant Item types, including all
CASE-derived classes.

Notice that some of the queries generate a result column with type
VARCHAR, even though they should be ENUM.

Change-Id: Iabda7dc4ecf1c2755508c720481b6a871a626a71