[cmake] Unbreak LLVM-Config.cmake / llvm_expand_dependencies.

The algorithm for sorting libraries in topological order, as
previously implemented, had a few issues:
  * It didn't make any sense.
  * It didn't actually sort libraries in topological order.
  * It hung on some inputs, e.g. "LLVMipo".

This commit replaces the old algorithm with a straightforward port
from llvm-config.cpp.

llvm-svn: 224554