allocator_arg and piecewise_construct should use {}

[StephanTLavavej]

N3797 20.7.2 [memory.syn] and 20.7.6 [allocator.tag] say:

constexpr allocator_arg_t allocator_arg = allocator_arg_t();

20.2 [utility] and 20.3.5 [pair.piecewise] say:

constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();

This obeys 8.5 [dcl.init]/7 "If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.", but it's verbose. With C++11 tech, we can do better:

constexpr allocator_arg_t allocator_arg{};

constexpr piecewise_construct_t piecewise_construct{};