Allocator-aware container and allocator_arg_t

[Bjorn Reese]

A container must provide constructors that takes an allocator as
argument to meet the allocator-aware container requirements.

There is also the std::allocator_arg_t tag that can be used to
disambiguate constructors with and without the allocator argument.

If a container cannot provide the T(Alloc) constructor directly due
to ambiguity, but instead provides the T(allocator_arg_t, Alloc)
constructor, does it meet the allocator-aware container requirements?