Function inner_ptr_of_unique

fn inner_ptr_of_unique<'tcx>(
    ecx: &MiriInterpCx<'tcx>,
    place: &PlaceTy<'tcx>,
) -> InterpResult<'tcx, PlaceTy<'tcx>>

Takes a place for a Unique and turns it into a place with the inner raw pointer. I.e. input is what you get from the visitor upon encountering an adt that is Unique, and output can be used by retag_ptr_inplace.