Remove redundant __constexpr_addressof function

Remove redundant __constexpr_addressof function

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