[libc++][test] Fix `MaybePOCCAAllocator` to finally meet the allocator requirements (#74960)

Found while running libc++'s test suite with MSVC's STL.

After @CaseyCarter's [LLVM-D118279](https://reviews.llvm.org/D118279)
https://github.com/llvm/llvm-project/commit/c5ba46ea1804dfefb22e6d2bb65ff1636d2cc8cd
"\[libcxx\]\[test\] `MaybePOCCAAllocator` should meet the
*Cpp17Allocator* requirements" followed by @philnik777's
[LLVM-D68365](https://reviews.llvm.org/D68365)
https://github.com/llvm/llvm-project/commit/98d3d5b5da66e3cf7807c23a0294280bb796466b
"\[libc++\] Implement [P1004R2](https://wg21.link/P1004R2) (`constexpr
std::vector`)", one more change is necessary.

MSVC's `constexpr vector` implementation noticed this because we always
rebind allocators.