[iOS][CI] Upgrade xcode version to 12.0 (#45677)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45677
Test Plan: Imported from OSS
Reviewed By: husthyc
Differential Revision: D24065647
Pulled By: xta0
fbshipit-source-id: f2535b1d93e58cf79e7075bf56b0613a3ded16eb
