libstdc++: Fix errors in <experimental/internet>

libstdc++: Fix errors in <experimental/internet>

libstdc++-v3/ChangeLog:

	* include/experimental/internet (address_v6::any): Avoid using
	memcpy in constexpr function.
	(address_v6::loopback): Likewise.
	(make_address_v6): Fix missing return statements on error paths.
	* include/experimental/io_context: Avoid -Wdangling-else
	warning.
	* testsuite/experimental/net/internet/address/v4/members.cc:
	Remove unused variables.
	* testsuite/experimental/net/internet/address/v6/members.cc:
	New test.