MDEV-18769 Assertion `fixed == 1' failed in Item_cond_or::val_int

    This bug is caused by pushdown from HAVING into WHERE.
    It appears because condition that is pushed wasn't fixed.

    It is also discovered that condition pushdown from HAVING into
    WHERE is done wrong. There is no need to build clones for some
    conditions that can be pushed. They can be simply moved from HAVING
    into WHERE without cloning.
    build_pushable_cond_for_having_pushdown(),
    remove_pushed_top_conjuncts_for_having() methods are changed.

    It is found that there is no transformation made for fields of
    pushed condition.
    field_transformer_for_having_pushdown transformer is added.

    New tests are added. Some comments are changed.

mysql-test/main/derived_cond_pushdown.result

@@ -8218,39 +8218,18 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "table": {
-      "table_name": "<subquery2>",
-      "access_type": "system",
-      "rows": 1,
-      "filtered": 100,
-      "materialized": {
-        "unique": 1,
+      "message": "Impossible WHERE"
+    },
+    "subqueries": [
+      {
         "query_block": {
           "select_id": 2,
           "table": {
             "message": "Impossible WHERE"
           }
         }
       }
-    },
-    "table": {
-      "table_name": "<derived3>",
-      "access_type": "ALL",
-      "rows": 2,
-      "filtered": 100,
-      "attached_condition": "v1.c = NULL",
-      "materialized": {
-        "query_block": {
-          "select_id": 3,
-          "table": {
-            "table_name": "t1",
-            "access_type": "ALL",
-            "rows": 2,
-            "filtered": 100,
-            "attached_condition": "t1.c = NULL"
-          }
-        }
-      }
-    }
+    ]
   }
 }
 DROP VIEW v1;

mysql-test/main/having.result

@@ -483,7 +483,7 @@ HAVING (table2.f2 = 8);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 Warnings:
-Note	1003	select `test`.`table1`.`f1` AS `f1`,7 AS `f2` from `test`.`t1` `table1` join `test`.`t1` `table2` where 0 group by `test`.`table1`.`f1`,7 having multiple equal(8, 7)
+Note	1003	select `test`.`table1`.`f1` AS `f1`,7 AS `f2` from `test`.`t1` `table1` join `test`.`t1` `table2` where 0 group by `test`.`table1`.`f1`,7 having 1
 DROP TABLE t1;
 #
 # Bug#52336 Segfault / crash in 5.1 copy_fields (param=0x9872980) at sql_select.cc:15355