Create new copies of distinct `MDNode`s instead of following the
uniquing `MDNode` logic.
Just like self-references (or other cycles), `MapMetadata()` creates a
new node. In practice most calls use `RF_NoModuleLevelChanges`, in
which case nothing is duplicated anyway.
Part of PR22111.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225476 91177308-0d34-0410-b5e6-96231b3b80d8
