[OpenMP][DeviceRTL][NFC] Use `OMPTgtExecModeFlags` from `llvm/include/llvm/Frontend...

[OpenMP][DeviceRTL][NFC] Use `OMPTgtExecModeFlags` from `llvm/include/llvm/Frontend/OpenMP/OMPDeviceConstants.h`

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