Use is_using_fission in legacy_fields_migrator

Since unknown commit is submitted, the principled variable to use in per_object_debug_info is is_using_fission, not per_object_debug_info_file (in case of thinlto build, the former is present also for thinlto bitcode compile, the latter only for backend compile).

RELNOTES: None.
PiperOrigin-RevId: 237192121

tools/migration/legacy_fields_migration_lib.py

@@ -126,6 +126,7 @@ def migrate_legacy_fields(crosstool):
         toolchain, "per_object_debug_info"):
       # feature {
       #   name: "per_object_debug_info"
+      #   enabled: true
       #   flag_set {
       #     action: "assemble"
       #     action: "preprocess-assemble"
@@ -134,7 +135,7 @@ def migrate_legacy_fields(crosstool):
       #     action: "c++-module-codegen"
       #     action: "lto-backend"
       #     flag_group {
-      #       expand_if_all_available: 'per_object_debug_info_file'",
+      #       expand_if_all_available: 'is_using_fission'",
       #       flag: "-gsplit-dwarf"
       #     }
       #   }
@@ -148,7 +149,7 @@ def migrate_legacy_fields(crosstool):
           "preprocess-assemble", "lto-backend"
       ]
       flag_group = flag_set.flag_group.add()
-      flag_group.expand_if_all_available[:] = ["per_object_debug_info_file"]
+      flag_group.expand_if_all_available[:] = ["is_using_fission"]
       flag_group.flag[:] = ["-gsplit-dwarf"]
 
     if toolchain.objcopy_embed_flag and not _get_feature(

tools/migration/legacy_fields_migration_lib_test.py

@@ -941,6 +941,9 @@ class LegacyFieldsMigrationLibTest(unittest.TestCase):
     output = crosstool.toolchain[0]
     self.assertEqual(output.feature[0].name, "per_object_debug_info")
     self.assertEqual(output.feature[0].enabled, True)
+    self.assertEqual(
+        output.feature[0].flag_set[0].flag_group[0].expand_if_all_available,
+        ["is_using_fission"])
 
   def test_supports_fission_not_migrated_on_false(self):
     crosstool = make_crosstool("supports_fission: false")