Remove duplicate hard coding of source and destination

[theunrepentantgeek]

Describe the current behavior

At the moment, the two construction methods for PropertyAssignmentFunction are explicitly adding source and destination into KnownLocals so that those identifiers aren't reused when generating local variables inside the function bodies.

Describe the improvement

Having those variables hard coded where they are duplicates the parameter naming already present, and makes the code brittle as a change in one place would need to be replicated in the other place in order to avoid problems.

To fix this, we need to modify the function generation so that parameter names are assigned (and reserved) in the same place.