ExternalProject: Restore support for Xcode with an effective platform
Revert commit cabad8a3 from !8169 (merged) and restore Xcode-specific behavior intentionally preserved by !7341 (merged). Unfortunately we still do not have a test case, so leave a comment to avoid reverting this.
Issue: #23645 (closed)
Issue: #23652
Backport: release
Manual-backport: release-3.28
Edited by Brad King