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

[David Spickett via llvm-commits]

DavidSpickett wrote:

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

User error on my part, it works fine and because it uses CMake's config mechanism to find clang it does fail in a reasonable way.

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