MDEV-19631: Assertion `0' failed in st_select_lex_unit::optimize or different plan upon 2nd execution of PS with EXPLAIN

Second execution of a prepared statement for a query containing a constant
subquery with union that can be optimized away, could result in server abnormal
termination for debug build or incorrect result set output for release build.

For example, the following test case crashes a server built with debug on second
run of the statement EXECUTE stmt
  CREATE TABLE t1 (a INT);
  PREPARE stmt FROM 'EXPLAIN SELECT * FROM t1 HAVING 6 IN ( SELECT 6 UNION SELECT 5 )';
  EXECUTE stmt;
  EXECUTE stmt;

The reason for incorrect result set output or abnormal server termination
is careless working with the data member fake_select_lex->options inside
the function mysql_explain_union(). Once the flag SELECT_DESCRIBE is set in
the data member fake_select_lex->option before calling the methods
  SELECT_LEX_UNIT::prepare/SELECT_LEX_UNIT::execute
the original value of the option is no longer restored.
As a consequence, next time the prepared statement is re-executed we have
the fake_select_lex with the flag SELECT_DESCRIBE set in the data member
fake_select_lex->option, that is incorrect. In result, the method
  Item_subselect::assigned()
is not invoked during evaluation of a constant condition (constant subquery
with union) that being performed on OPTIMIZE phase of query handling.

This leads to the fact that records in the temporary table are not deleted
before calling
  table->file->ha_enable_indexes(HA_KEY_SWITCH_ALL)
in the method st_select_lex_unit::optimize().
In result table->file->ha_enable_indexes(HA_KEY_SWITCH_ALL) returns error
and DBUG_ASSERT(0) is fired.

Stack trace to the line where the error generated on re-enabling indexes
for next subselect iteration is below:
st_select_lex_unit::optimize (at sql_union.cc:954)
  handler::ha_enable_indexes (at handler.cc:4338)
    ha_heap::enable_indexes (at ha_heap.cc:519)
      heap_enable_indexes (at hp_clear.c:164)

The code snippet to clarify raising the error is also listed:
int heap_enable_indexes(HP_INFO *info)
{
  int error= 0;
  HP_SHARE *share= info->s;

  if (share->data_length || share->index_length)
    error= HA_ERR_CRASHED; <<== set error the value HA_ERR_CRASHED
                                since share->data_length != 0

To fix this issue the original value of unit->fake_select_lex->options
has to be saved before setting the flag SELECT_DESCRIBE and restored
on return from invocation of SELECT_LEX_UNIT::prepare/SELECT_LEX_UNIT::execute