[Clang] [Docs] Add some CMake example code for linking against libclang (#166268)
Though we have a few code examples in our documentation that show how to
*use* libclang, we never actually show how to *link* against it. I
myself mostly figured this out through trial and error some time ago,
and I’ve since had to explain it to others on several occasions, so I
thought adding some very minimal CMake example code might be helpful.