MySQL provides what appears to be a non standard extension to the
FOREIGN KEY syntax which let users name (label/tag) a foreign key
to more easily identify a specific foreign key if any problems show
up later during the query parsing or execution. But the foreign key
name was not being properly set to the right key, possible leaving
the foreign key with no name.


mysql-test/include/mix1.inc:
  Add test case for Bug#30747
mysql-test/r/innodb_mysql.result:
  Add test case result for Bug#30747
sql/sql_yacc.yy:
  Set the foreign key name to the constraint name if a specific name was
  not provided. As for the constraint name, only use the foreign name if
  a specific name was not provided.