[llvm] Add asserts in (ThreadSafe)?RefCountedBase destructors

Added a trivial destructor in release mode and in debug mode a destructor that asserts RefCount is indeed zero.
This ensure people aren't manually (maybe accidentally) destroying these objects like in this contrived example.
```lang=c++
{
  std::unique_ptr<SomethingRefCounted> Object;
  holdIntrusiveOwnership(Object.get());
  // Object Destructor called here will assert.
}
```

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D92480