[cmake] Teach the Clang CMake build to use LLVM_LIBDIR_SUFFIX for its

'lib' directories in the build. This variable is available now both as
part of the normal LLVM build an as part of a standalone build as I've
added it to the LLVMConfig.cmake output.

With this change we should at least put libraries into the multilib
directory correctly. It is the first step in getting Clang to be
reasonably multilib aware.

llvm-svn: 224923