OpenMP `omp_get_mapped_hostptr` extension

[jdenny-ornl]

Clacc prototypes the following routine as an OpenMP extension to support OpenACC's acc_hostptr:

void *omp_get_mapped_hostptr(const void *ptr, int device_num);

The initial commit is dec7c17. This feature should be considered for upstreaming to LLVM as described in #10.