Problem: currently, UCS-2 cannot be used as a client character set.

Fix: raise an error if one attempts to set it to USC-2.


mysql-test/r/ctype_ucs.result:
  Fix for bug#31615: crash after set names ucs2 collate xxx
    - test result.
mysql-test/t/ctype_ucs.test:
  Fix for bug#31615: crash after set names ucs2 collate xxx
    - test case.
sql/set_var.cc:
  Fix for bug#31615: crash after set names ucs2 collate xxx
    - raise an error if one is going to set character_set_client to UCS-2.
sql/set_var.h:
  Fix for bug#31615: crash after set names ucs2 collate xxx
    - raise an error if one is going to set character_set_client to UCS-2.