[llvm] [clang] Fix darwin-related tests' XFAIL annotation (PR #130144)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

I don't have a Mac to test this on. It seems simple enough and I'm fairly confident it is correct. I assume you have verified that this does indeed XFAIL the tests under the condition you expect it to?

One request: please change the tag in the title to `[llvm-ar]`, since this change has nothing to do with clang. LGTM with that.

https://github.com/llvm/llvm-project/pull/130144