Make const local element (ce) cluster fields immutable

[spathiwa]

Currently if a cluster is declared with ce(), you can write to the whole cluster, but you CAN incorrectly write to a field.

I think fixing this efficiently would require refactoring GetVIElementAddressFromPath and GetSubElementAddressFromPath to have an extra isConstantP pointer parameter which is computed while traversing the path to be true if an element or any of its parents is constant.