Document the new LLVM CMake interface for building against LLVM
libraries. With many contributions from Brad King.
Emit a warning if llvm_map_components_to_libraries() is used noting that its
use is deprecated in favour of llvm_map_components_to_libnames()

Although message(DEPRECATION "msg") would probably be a better fit this
does nothing if CMAKE_ERROR_DEPRECATED and CMAKE_WARNING_DEPRECATED are
both off, which is the default.