[NFC][libc++] Mark LWG3296 as complete.

I recalled Marshall had already made this change. The change is
committed in e3f89a989a23b9bfcb9f9d01172cebb63db627e1.

GitOrigin-RevId: 83d26603e0402befe32186d2a2706bdf79d0a1cf
1 file changed