OpenMP update routine extensions

Clacc prototypes the following routines as OpenMP extensions to support OpenACC's `acc_update_device` and `acc_update_self`:

```
void omp_target_update_to(void *ptr, size_t size, int device_num);
void omp_target_update_from(void *ptr, size_t size, int device_num);
```

The initial commit is c9236faa4234.  This feature should be considered for upstreaming to LLVM as described in #10.