IR: Stop erasing MDNodes from uniquing sets during teardown

Stop erasing `MDNode`s from the uniquing sets in `LLVMContextImpl`
during teardown (in particular, during
`UniquableMDNode::~UniquableMDNode()`).  Although it's currently
feasible, there isn't any clear benefit and it may not be feasible for
other subclasses (which don't explicitly store the lookup hash).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225696 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Metadata.h

@@ -722,7 +722,7 @@ protected:
   /// resolveCycles() is called).
   UniquableMDNode(LLVMContext &C, unsigned ID, ArrayRef<Metadata *> Vals,
                   bool AllowRAUW);
-  ~UniquableMDNode();
+  ~UniquableMDNode() {}
 
   void storeDistinctInContext();
 
@@ -767,7 +767,7 @@ class MDTuple : public UniquableMDNode {
 
   MDTuple(LLVMContext &C, ArrayRef<Metadata *> Vals, bool AllowRAUW)
       : UniquableMDNode(C, MDTupleKind, Vals, AllowRAUW) {}
-  ~MDTuple();
+  ~MDTuple() { dropAllReferences(); }
 
   void setHash(unsigned Hash) { MDNodeSubclassData = Hash; }
   void recalculateHash();