IR: Allow temporary nodes to become uniqued or distinct

Add `MDNode::replaceWithUniqued()` and `MDNode::replaceWithDistinct()`,
which mutate temporary nodes to become uniqued or distinct.  On uniquing
collisions, the unique version is returned and the node is deleted.

This takes advantage of temporary nodes being folded back in, and should
let me clean up some awkward logic in `MapMetadata()`.

llvm-svn: 226510