Game/Maps: Instance handling follow-up:

- Fix a bug where a player could maintain a conflicting non-perm solo bind if they were in the instance when invited to group. Closes #16150.
- If a group is created while the leader is in an instance that nobody is bound to, the group will take over the instance and bind to it. This stops the homebind timer when reforming group after disconnects and the like.

(cherry picked from commit 0f0a51b87a)

src/server/game/Maps/Map.cpp

@@ -3031,7 +3031,7 @@ bool InstanceMap::AddPlayerToMap(Player* player)
             {
                 if (group)
                 {
-                    // solo saves should be reset when entering a group
+                    // solo saves should have been reset when the map was loaded
                     InstanceGroupBind* groupBind = group->GetBoundInstance(this);
                     if (playerBind && playerBind->save != mapSave)
                     {