[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.
Reviewed By: jdoerfert
Subscribers: yaxunl, guansong, openmp-commits
Differential Revision: https://reviews.llvm.org/D79255