[XLA:AOT] Alias the input and output buffers of converted variables

This tells buffer assignment to forcibly alias the buffers. From a user
perspective this means that the updates are carried out in-place, which is what we want.

The implementation just pushes aliasing information through a few layers, turns
out that the code using it is already in place (but unused) so this happens to
work.

PiperOrigin-RevId: 235883842