MDEV-34123 CONCAT Function Returns Unexpected Empty Set in Query

Search conditions were evaluated using val_int(), which was wrong.
Fixing the code to use val_bool() instead.

Details:
- Adding a new item_base_t::IS_COND flag which marks Items used
  as <search condition> in WHERE, HAVING, JOIN ON, CASE WHEN clauses.
  The flag is at the parse time.
  These expressions must be evaluated using val_bool() rather than val_int().

  Note, the optimizer creates more Items which are used as search conditions.
  Most of these items are not marked with IS_COND yet. This is OK for now,
  but eventually these Items can also be fixed to have the flag.

- Adding a method Item::is_cond() which tests if the Item has the IS_COND flag.

- Implementing Item_cache_bool. It evaluates the cached expression using
  val_bool() rather than val_int().
  Overriding Type_handler_bool::Item_get_cache() to create Item_cache_bool.

- Implementing Item::save_bool_in_field(). It uses val_bool() rather than
  val_int() to evaluate the expression.

- Implementing Type_handler_bool::Item_save_in_field()
  using Item::save_bool_in_field().

- Fixing all Item_bool_func descendants to implement a virtual val_bool()
  rather than a virtual val_int().

- To find places where val_int() should be fixed to val_bool(), a few
  DBUG_ASSERT(!is_cond()) where added into val_int() implementations
  of selected (most frequent) classes:

  Item_field
  Item_str_func
  Item_datefunc
  Item_timefunc
  Item_datetimefunc
  Item_cache_bool
  Item_bool_func
  Item_func_hybrid_field_type
  Item_basic_constant descendants

- Fixing all places where DBUG_ASSERT() happened during an "mtr" run
  to use val_bool() instead of val_int().

sql/sql_handler.cc

@@ -989,7 +989,7 @@ retry:
       }
       goto ok;
     }
-    if (cond && !cond->val_int())
+    if (cond && !cond->val_bool())
     {
       if (thd->is_error())
         goto err;