[llvm-branch-commits] [llvm] [libc++][CI] Pin the XCode version. (PR #135412)

[Nikolas Klauser via llvm-branch-commits]

https://github.com/philnik777 approved this pull request.

@ldionne Should make sure this is correct when he is back. I'm not entirely sure whether "latest stable release" means latest minor or latest major (also, we're not up-to-date in the docs which XCode we're supporting).

https://github.com/llvm/llvm-project/pull/135412