[mojo-lang] Fix MOCO-757: Transfer ^ of borrowed arg leads to double …

…free (#40892)

Mojo used to support transfering out of `let` declarations, which
were immutable but owned.  We don't have to support this anymore,
so there is no reason to support transfering from immutable refs.

MODULAR_ORIG_COMMIT_REV_ID: ee2358b31fb300ab0843b53f54eea065b55e0fda

stdlib/test/test_utils/types.mojo

@@ -22,7 +22,7 @@ struct MoveOnly[T: Movable](Movable):
     var data: T
     """Test data payload."""
 
-    fn __init__(inout self, i: T):
+    fn __init__(inout self, owned i: T):
         """Construct a MoveOnly providing the payload data.
 
         Args: