This patch makes preparation for a series that will enable per-kernel information used in both host and device runtime. Some variables/enums, such as `OMPTgtExecModeFlags`, have to be shared by both of them. A new header `OMPDeviceConstants.h` is added, containing code that will be shared by them. We will introduce more variables soon. Reviewed By: jdoerfert Differential Revision: https://reviews.llvm.org/D142320