Decide on modelling buffer donation #740

Buffer donation is an important technique for reducing memory pressure that we want to model in StableHLO.

When forking from MHLO, we have inherited two attributes to model buffer donation: ArgResultAlias and OutputOperandAlias. These attributes are: 1) very similar to each other due to unclear reasons, 2) account for tuples which is something that we want to remove from StableHLO in the long term (#598).

Let's clean this up.