Adding support for target in_reduction

Implementing target in_reduction by wrapping target task with host task with in_reduction and if clause. This is in compliance with OpenMP 5.0 section: 2.19.5.6.
So, this

```
  for (int i=0; i<N; i++) {
    res = res+i
  }
```

will become

```

   #pragma omp task in_reduction(+:res) if(0)
   #pragma omp target map(res)
   for (int i=0; i<N; i++) {
     res = res+i
   }
```

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D125669