MDEV-11357 Split Item_cache::get_cache() into virtual methods in Type_handler

This patch:
- Adds a new virtual method Type_handler::Item_get_cache
- Splits moves Item_cache::get_cache() into the new method, every
  "case XXX_RESULT" to the corresponding Type_handler_xxx::Item_get_cache.
- Adds Item::get_cache as a convenience wrapper, to make the caller code
  shorter.
- Changes the last argument of Arg_comparator::cache_converted_constant()
  from Item_result to "const Type_handler *".
- Removes subselect_engine::cmp_type, subselect_engine::res_type,
  subselect_engine::res_field_type and derives subselect_engine
  from Type_handler_hybrid_field_type instead.
- Makes Type_handler_varchar public, as it's now needed as the
  default data type handler for subselect_engine.

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/opt_subselect.cc b/sql/opt_subselect.cc
index a9222cbca42..461a2107437 100644
--- a/sql/opt_subselect.cc
+++ b/sql/opt_subselect.cc
@@ -5228,7 +5228,7 @@ int select_value_catcher::setup(List<Item> *items)
   List_iterator<Item> li(*items);
   for (uint i= 0; (sel_item= li++); i++)
   {
-    if (!(row[i]= Item_cache::get_cache(thd, sel_item)))
+    if (!(row[i]= sel_item->get_cache(thd)))
       return TRUE;
     row[i]->setup(thd, sel_item);
   }