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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Whoops, I clicked too soon, so I closed the revert PR, sorry for that noise.

I think this is a spurious failure unrelated to your changes, so you can ignore the issue.

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