re PR libstdc++/77288 (Std::experimental::optional::operator= implementation is broke...

re PR libstdc++/77288 (Std::experimental::optional::operator= implementation is broken in gcc 6.1)

	PR libstdc++/77288
	* include/std/optional (__is_optional_impl, __is_optional): Remove.
	(__converts_from_optional, __assigns_from_optional): New.
	(optional(_Up&&)): Use is_same instead of __is_optional.
	(optional(const optional<_Up>&)): Constrain with
	__converts_from_optional.
	(optional(optional<_Up>&&)): Likewise.
	(operator=(_Up&&)): Use is_same instead of __is_optional, check
	is_same and is_scalar.
	(operator=(const optional<_Up>&)): Constrain with
	__converts_from_optional and __assigns_from_optional.
	(operator=(optional<_Up>&&)): Likewise.
	* testsuite/20_util/optional/77288.cc: New.
	* testsuite/20_util/optional/cons/value.cc: Adjust.

From-SVN: r240324