Don't show repaired TGOs as dead.

We were never resetting the dead state for repaired SAMs. Rather than
tracking that manually, determine liveness from the number of units left
alive.

For building objectives where the group is not assigned to the TGO until
*mission* generation time we still need manual tracking.

(cherry picked from commit d3b1f6110f)

game/event/event.py

@@ -152,12 +152,10 @@ class Event:
 
             loss.group.units.remove(loss.unit)
             loss.group.units_losts.append(loss.unit)
-            if not loss.ground_object.alive_unit_count:
-                loss.ground_object.is_dead = True
 
     def commit_building_losses(self, debriefing: Debriefing) -> None:
         for loss in debriefing.building_losses:
-            loss.ground_object.is_dead = True
+            loss.ground_object.kill()
             self.game.informations.append(Information(
                 "Building destroyed",
                 f"{loss.ground_object.dcs_identifier} has been destroyed at "

game/theater/theatergroundobject.py

@@ -85,10 +85,13 @@ class TheaterGroundObject(MissionTarget):
         self.dcs_identifier = dcs_identifier
         self.airbase_group = airbase_group
         self.sea_object = sea_object
-        self.is_dead = False
         # TODO: There is never more than one group.
         self.groups: List[Group] = []
 
+    @property
+    def is_dead(self) -> bool:
+        return self.alive_unit_count == 0
+
     @property
     def units(self) -> List[Unit]:
         """
@@ -161,6 +164,9 @@ class BuildingGroundObject(TheaterGroundObject):
             sea_object=False
         )
         self.object_id = object_id
+        # Other TGOs track deadness based on the number of alive units, but
+        # buildings don't have groups assigned to the TGO.
+        self._dead = False
 
     @property
     def group_name(self) -> str:
@@ -171,6 +177,15 @@ class BuildingGroundObject(TheaterGroundObject):
     def waypoint_name(self) -> str:
         return f"{super().waypoint_name} #{self.object_id}"
 
+    @property
+    def is_dead(self) -> bool:
+        if not hasattr(self, "_dead"):
+            self._dead = False
+        return self._dead
+
+    def kill(self) -> None:
+        self._dead = True
+
 
 class NavalGroundObject(TheaterGroundObject):
     def mission_types(self, for_player: bool) -> Iterator[FlightType]: