infra: Upgrade XCode version to "12a7209". This mirrors the version used in Chromium. Bug: angleproject:5559 Change-Id: Ie1702281ce88fef00bf46079ae76477d33207dfd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2679298 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>