MDEV-28965 Assertion failure when preparing UPDATE with derived table in WHERE

This patch fixes not only the assertion failure in the function
Field_iterator_table_ref::set_field_iterator() but also:
 - fixes the problem of forced materialization of derived tables used
   in subqueries contained in WHERE clauses of single-table and multi-table
   UPDATE and DELETE statements
 - fixes the problem of MDEV-17954 that prevented execution of multi-table
   DELETE statements if they use in their WHERE clauses references to
   the tables that are updated.

The patch must be considered a complement to the patch for MDEV-28883.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

sql/sql_delete.h

@@ -44,11 +44,13 @@ class Sql_cmd_delete final : public Sql_cmd_dml
 {
 public:
   Sql_cmd_delete(bool multitable_arg)
-    :  multitable(multitable_arg), save_protocol(NULL) {}
+    : orig_multitable(multitable_arg), multitable(multitable_arg),
+      save_protocol(NULL)
+  {}
 
   enum_sql_command sql_command_code() const override
   {
-    return multitable ? SQLCOM_DELETE_MULTI : SQLCOM_DELETE;
+    return orig_multitable ? SQLCOM_DELETE_MULTI : SQLCOM_DELETE;
   }
 
   DML_prelocking_strategy *get_dml_prelocking_strategy()
@@ -56,6 +58,12 @@ public:
     return &dml_prelocking_strategy;
   }
 
+  bool processing_as_multitable_delete_prohibited(THD *thd);
+
+  bool is_multitable() { return multitable; }
+
+  void set_as_multitable() { multitable= true; }
+
 protected:
   /**
     @brief Perform precheck of table privileges for delete statements
@@ -78,6 +86,9 @@ protected:
   */
   bool delete_from_single_table(THD *thd);
 
+  /* Original value of the 'multitable' flag set by constructor */
+  const bool orig_multitable;
+
   /*
     True if the statement is a multitable delete or converted to such.
     For a single-table delete this flag is set to true if the statement