[llvm] [llvm][cmake] Error when a runtime is in LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES (PR #109791)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

That being said I do think it's a good warning to have in general, since I've seen users attempt to set both unknowingly. 

https://github.com/llvm/llvm-project/pull/109791