Constrain optional's __constexpr_addressof in its return type and use a constexpr...

Constrain optional's __constexpr_addressof in its return type and use a constexpr addressof for optional, if available.

Constrain optional's __constexpr_addressof in its return type
and use a constexpr addressof for optional, if available.
* include/experimental/optional (__constexpr_addressof):
Constrain in the return type instead of in a template parameter.
(_Has_addressof_mem)
(_Has_addressof_free, _Has_addressof, __constexpr_addressof):
Guard with #ifndef __cpp_lib_addressof_constexpr.
(operator->()): Use std::__addressof if it's constexpr.

From-SVN: r243298