Making it meet the requirements for allocator since C++11. Fixes
#113609.
This PR doesn't make it meet the C++03 allocator requirements, because
that would make the type too verbose and libc++ has backported many
C++11 features to the C++03 mode.
Drive-by: Removes the `TEST_CONSTEXPR_CXX14` on `allocate`/`dealocate`
which is never in effect (and causes IFNDR-ness before C++23), since
these functions modify the namespace-scoped variable `allocated_`.