[OpenMP][libomptarget] Add __tgt_target_return_t enum for __tgt_target_XXX return int

The defintion of OFFLOAD_SUCCESS and OFFLOAD_FAIL used in plugin APIs and libomptarget public APIs are not consistent.
Create __tgt_target_return_t for libomptarget public APIs.

Differential Revision: https://reviews.llvm.org/D109304

GitOrigin-RevId: 2187cbf56ff19c600e697ce762ba487134d99a84
2 files changed