IR: Move MDNode clone() methods from ValueMapper to MDNode, NFC

Now that the clone methods used by `MapMetadata()` don't do any
remapping (and return a temporary), they make more sense as member
functions on `MDNode` (and subclasses).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226541 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed