[OpenMP] Fix an issue of wrong return type of DeviceRTLTy::getNumOfDevices

Summary: There is a typo in DeviceRTLTy::getNumOfDevices that the type of its return value is bool. It will lead to a problem of wrong device number returned from omp_get_num_devices.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: yaxunl, guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D79255