[llvm] [cmake] Introduce LLVM_DISABLE_PROJECTS to easily exclude projects (PR #141535)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

What's the expected use-case for this? I don't think that we need a new option to save people a few characters.

https://github.com/llvm/llvm-project/pull/141535