The std::__addressof function is always constexpr, even in C++14, so we
can just use that.
* include/experimental/bits/lfts_config.h: Improve doc markup.
* include/experimental/optional: Improve docs.
(_Has_addressof_mem, _Has_addressof_free, _Has_addressof)
(__constexpr_addressof): Remove.
(optional::operator->()): Use std::__addressof().
* include/std/optional (optional::operator->()): Adjust whitespace.
* testsuite/experimental/optional/constexpr/observers/2.cc: Check
that operator-> is still constexpr with overloaded operator&. Change
to compile-only test.
* testsuite/experimental/optional/constexpr/observers/3.cc: Change to
compile-only test.
From-SVN: r270826
b752e2c926