[iOS][OSS][BE] Update XCode to use 12.5.1 (#64850)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64850
ghstack-source-id: 137827895
Test Plan: CircleCI
Reviewed By: hanton
Differential Revision: D30877964
fbshipit-source-id: 803f2506a755b3815024704e6177c7826bc42de8
