when they are destroyed, which makes Constant::destroyConstant an actually
useful external interface. Expose these methods publicly.
- Implement destroyConstant on ConstPointerNull so that destroyConstant can
be used on any derived type constant safely.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3378 91177308-0d34-0410-b5e6-96231b3b80d8