[flang] [acc] Adding allocation to the recipe of scalar allocatables (#154643)
Currently the privatization recipe of a scalar allocatable is as follow:
```
acc.private.recipe @privatization_ref_box_heap_i32 : !fir.ref<!fir.box<!fir.heap<i32>>> init {
^bb0(%arg0: !fir.ref<!fir.box<!fir.heap<i32>>>):
%0 = fir.alloca !fir.box<!fir.heap<i32>>
%1:2 = hlfir.declare %0 {uniq_name = "acc.private.init"} : (!fir.ref<!fir.box<!fir.heap<i32>>>) -> (!fir.ref<!fir.box<!fir.heap<i32>>>, !fir.ref<!fir.box<!fir.heap<i32>>>)
acc.yield %1#0 : !fir.ref<!fir.box<!fir.heap<i32>>>
}
```
This change adds the allocation for the scalar.