MDEV-4265 5.5 is slower than 5.3 because of many str_to_datetime calls

get_datetime_value() should not double-cache its own Item_cache_temporal items,
but it *should* cache other Item_cache items, such as Item_cache_str.

sql/item.h:
  shortcut, to avoid going through the switch in Item::cmp_type()
sql/item_cmpfunc.cc:
  even if the item is Item_cache_str - it still needs to be converted and cached.
sql/item_timefunc.h:
  all descendants of Item_temporal_func always have cmp_type==TIME_RESULT.
  Even Item_date_add_interval, that might have field_type == MYSQL_TYPE_STRING.

mysql-test/r/cache_temporal_4265.result

+create table t1 (a date);
+insert t1 values ('2000-01-02'), ('2001-02-03'), ('2002-03-04');
+set debug_dbug='d,str_to_datetime_warn';
+select * from t1 where a > date_add('2000-01-01', interval 5 day);
+a
+2001-02-03
+2002-03-04
+Warnings:
+Note	1003	2000-01-01
+Note	1003	2000-01-06
+drop table t1;

mysql-test/t/cache_temporal_4265.test

+#
+# MDEV-4265 5.5 is slower than 5.3 because of many str_to_datetime calls
+#
+--source include/have_debug.inc
+
+create table t1 (a date);
+insert t1 values ('2000-01-02'), ('2001-02-03'), ('2002-03-04');
+set debug_dbug='d,str_to_datetime_warn';
+select * from t1 where a > date_add('2000-01-01', interval 5 day);
+drop table t1;
+

sql/item.h

@@ -4030,6 +4030,7 @@ public:
   bool cache_value();
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
   int save_in_field(Field *field, bool no_conversions);
+  Item_result cmp_type() const { return TIME_RESULT; }
   void store_packed(longlong val_arg, Item *example);
   /*
     Having a clone_item method tells optimizer that this object

sql/item_cmpfunc.cc

@@ -906,7 +906,8 @@ get_datetime_value(THD *thd, Item ***item_arg, Item **cache_arg,
   }
   if ((*is_null= item->null_value))
     return ~(ulonglong) 0;
-  if (cache_arg && item->const_item() && item->type() != Item::CACHE_ITEM)
+  if (cache_arg && item->const_item() &&
+      !(item->type() == Item::CACHE_ITEM && item->cmp_type() == TIME_RESULT))
   {
     Query_arena backup;
     Query_arena *save_arena= thd->switch_to_arena_for_cached_items(&backup);

sql/item_timefunc.h

@@ -491,6 +491,7 @@ public:
   enum Item_result result_type () const { return STRING_RESULT; }
   CHARSET_INFO *charset_for_protocol(void) const { return &my_charset_bin; }
   enum_field_types field_type() const { return MYSQL_TYPE_DATETIME; }
+  Item_result cmp_type() const { return TIME_RESULT; }
   String *val_str(String *str);
   longlong val_int();
   double val_real();

sql/sql_time.cc

@@ -302,6 +302,9 @@ str_to_datetime_with_warn(CHARSET_INFO *cs,
     make_truncated_value_warning(thd, MYSQL_ERROR::WARN_LEVEL_WARN,
                                  str, length, flags & TIME_TIME_ONLY ?
                                  MYSQL_TIMESTAMP_TIME : ts_type, NullS);
+  DBUG_EXECUTE_IF("str_to_datetime_warn",
+                  push_warning(thd, MYSQL_ERROR::WARN_LEVEL_NOTE,
+                               ER_YES, str););
   return ts_type;
 }