[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