[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.

Author: Renaud-K

flang/lib/Optimizer/OpenACC/Support/FIROpenACCTypeInterfaces.cpp

@@ -614,6 +614,11 @@ mlir::Value OpenACCMappableModel<Ty>::generatePrivateInit(
     mlir::Type innerTy = fir::unwrapRefType(boxTy.getEleTy());
     if (fir::isa_trivial(innerTy)) {
       retVal = getDeclareOpForType(unwrappedTy).getBase();
+      mlir::Value allocatedScalar =
+          fir::AllocMemOp::create(builder, loc, innerTy);
+      mlir::Value firClass =
+          fir::EmboxOp::create(builder, loc, boxTy, allocatedScalar);
+      fir::StoreOp::create(builder, loc, firClass, retVal);
     } else if (mlir::isa<fir::SequenceType>(innerTy)) {
       hlfir::Entity source = hlfir::Entity{var};
       auto [temp, cleanup] = hlfir::createTempFromMold(loc, firBuilder, source);

flang/test/Lower/OpenACC/acc-private.f90

@@ -95,6 +95,9 @@
 ! CHECK: ^bb0(%arg0: !fir.ref<!fir.box<!fir.heap<i32>>>):
 ! CHECK:   %[[ALLOCA:.*]] = fir.alloca !fir.box<!fir.heap<i32>>
 ! CHECK:   %[[DECLARE:.*]]:2 = hlfir.declare %[[ALLOCA]] {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>>>)
+! CHECK:   %[[ALLOCMEM:.*]] = fir.allocmem i32
+! CHECK:   %[[BOX:.*]] = fir.embox %[[ALLOCMEM]] : (!fir.heap<i32>) -> !fir.box<!fir.heap<i32>>
+! CHECK:   fir.store %[[BOX]] to %[[DECLARE]]#0 : !fir.ref<!fir.box<!fir.heap<i32>>>
 ! CHECK:   acc.yield %[[DECLARE]]#0 : !fir.ref<!fir.box<!fir.heap<i32>>>
 ! CHECK: }