[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)
c5ba46e
"\[libcxx\]\[test\] `MaybePOCCAAllocator` should meet the
*Cpp17Allocator* requirements" followed by @philnik777's
[LLVM-D68365](https://reviews.llvm.org/D68365)
98d3d5b
"\[libc++\] 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.

Author: StephanTLavavej

libcxx/test/support/allocators.h

@@ -209,7 +209,7 @@ class MaybePOCCAAllocator {
         : id_(id), copy_assigned_into_(copy_assigned_into) {}
 
     template <class U>
-    MaybePOCCAAllocator(const MaybePOCCAAllocator<U, POCCAValue>& that)
+    TEST_CONSTEXPR MaybePOCCAAllocator(const MaybePOCCAAllocator<U, POCCAValue>& that)
         : id_(that.id_), copy_assigned_into_(that.copy_assigned_into_) {}
 
     MaybePOCCAAllocator(const MaybePOCCAAllocator&) = default;