re PR libstdc++/65978 (missing constexpr on std::forward_as_tuple and std::tie (LWG issues 2275 and 2301))

	PR libstdc++/65978
	* include/std/tuple (forward_as_tuple, tie): Add constexpr.
	* testsuite/20_util/tuple/creation_functions/constexpr.cc: Uncomment
	and fix tests for forward_as_tuple and tie.

From-SVN: r222719