libstdc++: Initialize all members of basic_endpoint union [PR109482]

libstdc++: Initialize all members of basic_endpoint union [PR109482]

On Solaris the in_addr struct contains a union and value-initializing it
does not make the s_addr member active. This means we can't access that
member later during constant evaluation.

Make the constructors explicitly set every member that we might want to
read later in constexpr member functions. This means even the default
constructor can only be constexpr for C++20, because we can't change the
active member of a union in older standards.

libstdc++-v3/ChangeLog:

	PR libstdc++/109482
	* include/experimental/internet (basic_endpoint::basic_endpoint()):
	Ensure that the required union members are active. Only define
	as constexpr for C++20 and later.
	(basic_endpoint::basic_endpoint(const protocol_type&, port_type)):
	Likewise.
	* testsuite/experimental/net/internet/endpoint/cons.cc: Only
	check constexpr default constructor for C++20 and later.
	* testsuite/experimental/net/internet/endpoint/extensible.cc:
	Likewise.