[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

Author: ye-luo

openmp/libomptarget/include/omptarget.h

@@ -28,6 +28,18 @@
 // Don't format out enums and structs.
 // clang-format off
 
+/// return flags of __tgt_target_XXX public APIs
+enum __tgt_target_return_t : int {
+  /// successful offload executed on a target device
+  OMP_TGT_SUCCESS = 0,
+  /// offload may not execute on the requested target device
+  /// this scenario can be caused by the device not available or unsupported
+  /// as described in the Execution Model in the specifcation
+  /// this status may not be used for target device execution failure
+  /// which should be handled internally in libomptarget
+  OMP_TGT_FAIL = ~0
+};
+
 /// Data attributes for each data reference used in an OpenMP target region.
 enum tgt_map_type {
   // No flags

openmp/libomptarget/src/interface.cpp

@@ -284,7 +284,7 @@ EXTERN int __tgt_target_mapper(ident_t *loc, int64_t device_id, void *host_ptr,
      DPxPTR(host_ptr), device_id);
   if (checkDeviceAndCtors(device_id, loc)) {
     DP("Not offloading to device %" PRId64 "\n", device_id);
-    return OFFLOAD_FAIL;
+    return OMP_TGT_FAIL;
   }
 
   if (getInfoLevel() & OMP_INFOTYPE_KERNEL_ARGS)
@@ -307,7 +307,8 @@ EXTERN int __tgt_target_mapper(ident_t *loc, int64_t device_id, void *host_ptr,
   if (rc == OFFLOAD_SUCCESS)
     rc = AsyncInfo.synchronize();
   handleTargetOutcome(rc == OFFLOAD_SUCCESS, loc);
-  return rc;
+  assert(rc == OFFLOAD_SUCCESS && "__tgt_target_mapper unexpected failure!");
+  return OMP_TGT_SUCCESS;
 }
 
 EXTERN int __tgt_target_nowait_mapper(
@@ -357,7 +358,7 @@ EXTERN int __tgt_target_teams_mapper(ident_t *loc, int64_t device_id,
      DPxPTR(host_ptr), device_id);
   if (checkDeviceAndCtors(device_id, loc)) {
     DP("Not offloading to device %" PRId64 "\n", device_id);
-    return OFFLOAD_FAIL;
+    return OMP_TGT_FAIL;
   }
 
   if (getInfoLevel() & OMP_INFOTYPE_KERNEL_ARGS)
@@ -380,7 +381,9 @@ EXTERN int __tgt_target_teams_mapper(ident_t *loc, int64_t device_id,
   if (rc == OFFLOAD_SUCCESS)
     rc = AsyncInfo.synchronize();
   handleTargetOutcome(rc == OFFLOAD_SUCCESS, loc);
-  return rc;
+  assert(rc == OFFLOAD_SUCCESS &&
+         "__tgt_target_teams_mapper unexpected failure!");
+  return OMP_TGT_SUCCESS;
 }
 
 EXTERN int __tgt_target_teams_nowait_mapper(