IR: Move call to dropAllReferences() to MDNode subclasses

Don't call `dropAllReferences()` from `MDNode::~MDNode()`, call it
directly from `~MDNodeFwdDecl()` and `~GenericMDNode()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223904 91177308-0d34-0410-b5e6-96231b3b80d8