[libc++][test] LWG2593: Moved-from state of Allocators (#107344)
The resolution of LWG2593 didn't require the standard library
implementation to change. It merely strengthened requirements on
user-defined allocator types and allowed the implementation to make
stronger assumptions. The status is tentatively set to Nothing To Do.
However, `test_allocator` in libc++'s test suit needs to be fixed to
conform to the strengthened requirements.
Closes #100220.