We should disable const subselect item evaluation because
subselect transformation does not happen in view_prepare_mode
and thus val_...() methods can not be called.


mysql-test/r/ctype_ucs.result:
  test case
mysql-test/r/view.result:
  test case
mysql-test/t/ctype_ucs.test:
  test case
mysql-test/t/view.test:
  test case
sql/item.cc:
  disabled const subselect item evaluation in
  view prepare mode.
sql/item_subselect.cc:
  added Item_subselect::safe_charset_converter which
  prevents const item evaluation in view prepare mode.
sql/item_subselect.h:
  added Item_subselect::safe_charset_converter which
  prevents const item evaluation in view prepare mode.