Use proper macro for unreachable switch cases

Those are the `default` cases which are not expected to hit (nor reachable) normally.

When the code is compiled with release mode and it reaches `Q_UNREACHABLE()`, it becomes
undefined behavior. So it rely on the developers to catch the errors in debug mode.
The upside of this is that the `switch` statement will be more optimized than not using it.
This also means the statements after `Q_UNREACHABLE()` isn't important. It allow anything to
preserve the intention of the code.

This macro is preferred over C++23 `std::unreachable` because it will automatically insert a
`Q_ASSERT(false)` with it.

PR #21752.

src/base/bittorrent/sessionimpl.cpp

@@ -296,14 +296,15 @@ namespace
     {
         switch (mode)
         {
-        default:
-            Q_ASSERT(false);
         case MoveStorageMode::FailIfExist:
             return lt::move_flags_t::fail_if_exist;
         case MoveStorageMode::KeepExistingFiles:
             return lt::move_flags_t::dont_replace;
         case MoveStorageMode::Overwrite:
             return lt::move_flags_t::always_replace_files;
+        default:
+            Q_UNREACHABLE();
+            break;
         }
     }
 }