libstdc++: Fix constexpr functions in <experimental/internet>

Change ip::basic_endpoint to work in constant expressions, but only for
C++20 and later (due to the use of a union, which cannot change active
member in constexpr evaluation until C++20).

During constant evaluation we cannot inspect the common initial sequence
of basic_endpoint's union members to check whether sin_family == AF_INET
or AF_INET6.  This means we need to store an additional boolean member
that remembers whether we have a v4 or v6 address. The address type can
change behind our backs if a user copies an address to the data()
pointer and then calls resize(n), so we need to inspect the sa_family_t
member in the union after a resize and update the boolean. POSIX only
guarantees that the sa_family_t member of each protocol-specific address
structure is at the same offset and of the same type, not that there is
a common initial sequence. The check in resize is done using memcmp, so
that we avoid accessing an inactive member of the union if the
sockaddr_in and sockaddr_in6 structures do not have a common initial
sequence that includes the sa_family_t member.

libstdc++-v3/ChangeLog:

	* include/experimental/internet (ip::make_address): Implement
	missing overload.
	(ip::address_v4::broadcast()): Avoid undefined shift.
	(ip::basic_endpoint): Fix member functions for constexpr.
	(ip::basic_endpoint::_M_is_v6): Replace member function with
	data member, adjust member functions using it.
	(ip::basic_endpoint::resize): Update _M_is_v6 based on sockaddr
	content.
	* testsuite/experimental/net/internet/address/v4/cons.cc: Fix
	constexpr checks to work in C++14.
	* testsuite/experimental/net/internet/address/v4/creation.cc:
	Likewise.
	* testsuite/experimental/net/internet/endpoint/cons.cc:
	Likewise.
	* testsuite/experimental/net/internet/network/v4/cons.cc:
	Likewise.
	* testsuite/experimental/net/internet/network/v4/members.cc:
	Likewise.
	* testsuite/experimental/net/internet/endpoint/extensible.cc: New test.

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