IR: MDNode => Value: AsmWriter SlotTracker API

Change `SlotTracker::CreateMetadataSlot()` and
`SlotTracker::getMetadataSlot()` to use `Value` instead of `MDNode`.
Part of PR21433.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221373 91177308-0d34-0410-b5e6-96231b3b80d8