[clang] Fix a cmake error when using the Xcode generator. (PR #119403)

[Aaron Ballman via cfe-commits]

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

LGTM but I added our cmake code owners just in case they have the bandwidth to also double-check, since I got this wrong before. :-)

https://github.com/llvm/llvm-project/pull/119403