Provide basic type safety for array_pod_sort comparators.

This makes using array_pod_sort significantly safer. The implementation relies
on function pointer casting but that should be safe as we're dealing with void*
here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191175 91177308-0d34-0410-b5e6-96231b3b80d8