[OpenMP][Offloading] Fix the issue that omp_get_num_devices returns wrong...

[OpenMP][Offloading] Fix the issue that omp_get_num_devices returns wrong number of devices, by Shiley Tian.

Summary:
This patch is to fix issue in the following simple case:

  #include <omp.h>
  #include <stdio.h>

  int main(int argc, char *argv[]) {
    int num = omp_get_num_devices();
    printf("%d\n", num);

    return 0;
  }

Currently it returns 0 even devices exist. Since this file doesn't contain any
target region, the host entry is empty so further actions like initialization
will not be proceeded, leading to wrong device number returned by runtime
function call.

Reviewers: jdoerfert, ABataev, protze.joachim

Reviewed By: ABataev

Subscribers: protze.joachim

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D72576