[cmake] Push LLVM_LIBDIR_SUFFIX through to the LLVMConfig.cmake file

that is used by other projects to build against LLVM. This will allow
subsequent patches to them to use LLVM_LIBDIR_SUFFIX, both when built as
part of the larger LLVM build an as part of a standalone build against
an installed set of LLVM libraries.

llvm-svn: 224920