The uncacheable flag should be set at fix_fields() stage.
Fixed by moving the flag setting to match the one in 5.1


mysql-test/r/query_cache.result:
  Bug #31157: test case
mysql-test/t/query_cache.test:
  Bug #31157: test case
sql/item_func.cc:
  Bug #31157: The uncacheable flag should be set at
  fix_fields() stage.
sql/item_func.h:
  Bug #31157: The uncacheable flag should be set at
  fix_fields() stage.