Invalid date like 2000-02-32 wasn't converted to int, which lead to not
using index and comparison with field as astring, which results in slow
query execution.

convert_constatn_item() and get_mm_leaf() now forces MODE_INVALID_DATES to
allow such conversion.


sql/item.h:
  Fix bug #14093 Query takes a lot of time when date format is not valid
  To Item_int_with_ref added method real_item() which returns ref.
sql/item_cmpfunc.cc:
   Fix bug #14093 Query takes a lot of time when date format is not valid
  convert_constant_item() now allows conversion of invalid dates like 2000-01-32 to int to make it possible to use index when comparing fields with such dates.
sql/opt_range.cc:
   Fix bug #14093 Query takes a lot of time when date format is not valid
  get_mm_leaf() modified so it allows index usage for comparing fields with invalid dates like 2000-01-32.
mysql-test/r/select.result:
  Test case for bug#14093 Query takes a lot of time when date format is not valid
mysql-test/t/select.test:
  Test case for bug#14093 Query takes a lot of time when date format is not valid