IR: Merge UniquableMDNode back into MDNode, NFC

As pointed out in r226501, the distinction between `MDNode` and
`UniquableMDNode` is confusing.  When we need subclasses of `MDNode`
that don't use all its functionality it might make sense to break it
apart again, but until then this makes the code clearer.

llvm-svn: 226520
diff --git a/llvm/lib/IR/LLVMContextImpl.cpp b/llvm/lib/IR/LLVMContextImpl.cpp
index 2fa6780..0c50a12 100644
--- a/llvm/lib/IR/LLVMContextImpl.cpp
+++ b/llvm/lib/IR/LLVMContextImpl.cpp
@@ -90,7 +90,7 @@
     Pair.second->dropUse();
 
   // Destroy MDNodes.
-  for (UniquableMDNode *I : DistinctMDNodes)
+  for (MDNode *I : DistinctMDNodes)
     I->deleteAsSubclass();
   for (MDTuple *I : MDTuples)
     delete I;