[llvm] [CMake] Do not use llvm-rc without Clang in external projects (PR #224505)

[Lang Hames via llvm-commits]

https://github.com/lhames commented:

Looks reasonable to me, but someone who knows the project CMake better should probably review too. @petrhosek @llvm-beanz ?

https://github.com/llvm/llvm-project/pull/224505