MDEV-24860: Incorrect behaviour of SET STATEMENT in case it is executed as a prepared statement

Running statements with SET STATEMENT FOR clause is handled incorrectly in
case the whole statement is executed in prepared statement mode.
For example, running of the following statement
  SET STATEMENT sql_mode = 'NO_ENGINE_SUBSTITUTION' FOR CREATE TABLE t1 AS SELECT CONCAT('abc') AS c1;
results in different definition of the table t1 depending on whether
the statement is executed as a prepared or as a regular statement.

In first case the column c1 is defined as
  `c1` varchar(3) DEFAULT NULL
in the last case the column c1 is defined as
  `c1` varchar(3) NOT NULL

Different definition for the column c1 arise due to the fact that
a value of the data memeber Item_func_concat::maybe_null depends on
whether strict mode is on or off. Below is definition of the method
fix_fields() of the class Item_str_func that is base class for the
class Item_func_concat that is created on parsing the
SET STATEMENT FOR clause.

bool Item_str_func::fix_fields(THD *thd, Item **ref)
{
  bool res= Item_func::fix_fields(thd, ref);
  /*
    In Item_str_func::check_well_formed_result() we may set null_value
    flag on the same condition as in test() below.
  */
  maybe_null= maybe_null || thd->is_strict_mode();
  return res;
}

Although the clause SET STATEMENT sql_mode = 'NO_ENGINE_SUBSTITUTION' FOR
is parsed on PREPARE phase during processing of the prepared statement,
real setting of the sql_mode system variable is done on EXECUTION phase.
On the other hand, the method Item_str_func::fix_fields is called on PREPARE
phase. In result, thd->is_strict_mode() returns true during calling the method
Item_str_func::fix_fields(), the data member maybe_null is assigned the value
true and column c1 is defined as DEFAULT NULL.

To fix the issue the system variables listed in the SET STATEMENT FOR clause
are set at the beginning of handling the PREPARE phase just right before
calling  the function check_prepared_statement() and their original values
restored immediate after return from this function.

Additionally, to avoid code duplication the source code used in the function
mysql_execute_command for setting variables, specified by SET STATEMENT
clause, were extracted to the standalone functions
run_set_statement_if_requested(). This new function is called from
the function  mysql_execute_command() and the method
Prepared_statement::prepare().

sql/sql_parse.h

@@ -101,6 +101,7 @@ void mysql_init_multi_delete(LEX *lex);
 bool multi_delete_set_locks_and_link_aux_tables(LEX *lex);
 void create_table_set_open_action_and_adjust_tables(LEX *lex);
 pthread_handler_t handle_bootstrap(void *arg);
+bool run_set_statement_if_requested(THD *thd, LEX *lex);
 int mysql_execute_command(THD *thd);
 bool do_command(THD *thd);
 void do_handle_bootstrap(THD *thd);