[llvm] [llvm][cmake] Add clang if not already present when building lldb (PR #149055)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

LGTM. IIUC this works for the standalone build because when building LLDB, we point it to `llvm-project/lldb` as the CMake root and don't specify `LLVM_ENABLE_PROJECTS`

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