libstdc++: Define net::socket_base::message_flags operators as friends [PR 100285]

libstdc++: Define net::socket_base::message_flags operators as friends [PR 100285]

The overloaded operators for socket_base::message_flags should only be
defined when the message_flags type itself is defined. Rather than
duplicate the preprocessor conditional, this moves the operators into
the same scope as the type, defining them as hidden friends.

As well as fixing the bug, this has all the usual advantages of hidden
friends (they are not visible to normal name lookup for unrelated
types).

For consistency, do the same for the resolver_base::flags bitmask
operators too.

libstdc++-v3/ChangeLog:

	PR libstdc++/100285
	* include/experimental/internet (resolver_base::flags):
	Define overloaded operators as hidden friends.
	* include/experimental/socket (socket_base::message_flags):
	Likewise.