MDEV-28201: Server crashes upon SHOW ANALYZE/EXPLAIN FORMAT=JSON

- Describe the lifetime of EXPLAIN data structures in
  sql_explain.h:ExplainDataStructureLifetime.

- Make Item_field::set_field() call set_refers_to_temp_table()
  when it refers to a temp. table.
- Introduce QT_DONT_ACCESS_TMP_TABLES flag for Item::print.
  It directs Item_field::print to not try access its the
  temp table.
- Introduce Explain_query::notify_tables_are_closed()
  and call it right before the query closes its tables.
- Make Explain data stuctures' print_explain_json() methods
  accept "no_tmp_tbl" parameter which means pass
  QT_DONT_ACCESS_TMP_TABLES when printing items.
- Make Show_explain_request::call_in_target_thread() not call
  set_current_thd(). This wasn't needed as the code inside
  lex->print_explain() uses output->thd anyway. output->thd
  refers to the SHOW command's THD object.

sql/sql_base.cc

@@ -759,6 +759,18 @@ close_all_tables_for_name(THD *thd, TABLE_SHARE *share,
 }
 
 
+int close_thread_tables_for_query(THD *thd)
+{
+  if (thd->lex && thd->lex->explain)
+    thd->lex->explain->notify_tables_are_closed();
+
+  DBUG_EXECUTE_IF("explain_notify_tables_are_closed",
+                  if (dbug_user_var_equals_str(thd, "show_explain_probe_query",
+                                               thd->query()))
+                      dbug_serve_apcs(thd, 1);
+                 );
+  return close_thread_tables(thd);
+}
 /*
   Close all tables used by the current substatement, or all tables
   used by this thread if we are on the upper level.