BUG#21658: Crash when creating table with item in prepared statement that allocates memory in fix_fields
We need to use an arena to indicate we are preparing a statement when loading partition function and
parsing it as part of an open table.


mysql-test/r/partition.result:
  Moved test case
mysql-test/t/partition.test:
  Moved test case
sql/item.cc:
  Shouldn't call change_item_tree in prepare statement phase even if arena was already activated
sql/sql_partition.cc:
  We need to use an arena to indicate we are preparing a statement when loading partition function and
  parsing it as part of an open table.
sql/table.cc:
  We need to use an arena to indicate we are preparing a statement when loading partition function and
  parsing it as part of an open table.