IR: Extract out and reuse storeImpl(), NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226499 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Metadata.h

@@ -820,6 +820,8 @@ protected:
   ~UniquableMDNode() {}
 
   void storeDistinctInContext();
+  template <class T, class StoreT>
+  static T *storeImpl(T *N, StorageType Storage, StoreT &Store);
 
 public:
   static bool classof(const Metadata *MD) {