[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:

If you use `LLVM_ENABLE_RUNTIMES=libc` it will automatically put `LLVM_ENABLE_PROJECTS=libc`, we use this to bootstrap some `libc` specific utilities. This patch breaks any runtimes build of the libc project. 

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