MDEV-29447 WIP Refactor spider_db_mbase_util::open_item_func

WIP: incorporating comments from
https://lists.launchpad.net/maria-developers/msg13283.html

spider_db_mbase_util::open_item_func() is a monster function.
It is difficult to maintain while it is expected that we need to
modify it when a new SQL function or a new func_type is added.

We split the function into two distinct functions: one handles the
case of str != NULL and the other handles the case of str == NULL.

This refactoring was done in a conservative way because we do not
have comprehensive tests on the function.

It also fixes a problem (MDEV-29447) where field items that are
arguments of a func item may be used before created / initialised.

Note this commit is a port of
3836098c (MDEV-26285) to current
versions 10.3+.
Signed-off-by: Yuchen Pei <yuchen.pei@mariadb.com>

storage/spider/mysql-test/spider/bugfix/t/mdev_29447.cnf

+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_29447.test

+--echo #
+--echo # MDEV-29447 SIGSEGV in spider_db_open_item_field and SIGSEGV spider_db_print_item_type, on SELECT
+--echo #
+ 
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+ 
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+eval CREATE TABLE tbl_a (
+    a INT
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+ 
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+eval CREATE TABLE tbl_a (
+    a INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a", srv "s_2_1"';
+ 
+SELECT TRIM(LEADING 'c' FROM a) FROM tbl_a;
+ 
+--connection child2_1
+DROP DATABASE auto_test_remote;
+ 
+--connection master_1
+DROP DATABASE auto_test_local;
+ 
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log

storage/spider/spd_db_mysql.h

@@ -99,6 +99,25 @@ class spider_db_mbase_util: public spider_db_util
     bool use_fields,
     spider_fields *fields
   );
+protected:
+  int check_item_func(
+    Item_func *item_func,
+    ha_spider *spider,
+    const char *alias,
+    uint alias_length,
+    bool use_fields,
+    spider_fields *fields
+  );
+  int print_item_func(
+    Item_func *item_func,
+    ha_spider *spider,
+    spider_string *str,
+    const char *alias,
+    uint alias_length,
+    bool use_fields,
+    spider_fields *fields
+  );
+public:
 #ifdef HANDLER_HAS_DIRECT_AGGREGATE
   int open_item_sum_func(
     Item_sum *item_sum,