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

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

I'm okay with merging this and keeping an eye on https://ci.swift.org/view/all/job/llvm.org/view/LLDB/job/lldb-cmake-standalone/ which covers all the standalone scenarios we officially support. 

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