OpenMP mapping routine extensions

[jdenny-ornl]

Clacc prototypes the following routines as OpenMP extensions to support OpenACC's data-clause-like data-management routines, such as acc_copyin:

void *omp_target_map_to(void *ptr, size_t size, int device_num);
void *omp_target_map_alloc(void *ptr, size_t size, int device_num);
void omp_target_map_from(void *ptr, size_t size, int device_num);
void omp_target_map_from_delete(void *ptr, size_t size, int device_num);
void omp_target_map_release(void *ptr, size_t size, int device_num);
void omp_target_map_delete(void *ptr, size_t size, int device_num);

The initial commit is 1275c34f496c. This feature should be considered for upstreaming to LLVM as described in #10.