OpenMP device-type routine extensions

[jdenny-ornl]

Clacc prototypes the following type and routines as OpenMP extensions to support OpenACC's type-based device-management routines, such as acc_get_device_type:

omp_device_t
omp_device_t omp_get_device_type(int device_num);
int omp_get_num_devices_of_type(omp_device_t device_type);
int omp_get_typed_device_num(int device_num);
int omp_get_device_of_type(omp_device_t device_type, int typed_device_num);

The initial commit is 608082a. This feature should be considered for upstreaming to LLVM as described in #10.