Disallow null as a named metadata operand.

Disallow null as a named metadata operand.
Make MDNode::destroy private.
Fix the one thing that used MDNode::destroy, outside of MDNode itself.

One should never delete or destroy an MDNode explicitly. MDNodes
implicitly go away when there are no references to them (implementation
details aside).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109028 91177308-0d34-0410-b5e6-96231b3b80d8