[llvm] [llvm][cmake] Error if lldb project is enabled without clang (PR #149055)

[David Spickett via llvm-commits]

DavidSpickett wrote:

You might wonder what a standalone build does when clang isn't in the base build, I also wonder this because I have not been able to get it to build at all :)

So I might do a follow up for standalone if I can work around the problems.

https://github.com/llvm/llvm-project/pull/149055