PR libstdc++/69608 Move semantics for strstreambuf

PR libstdc++/69608 Move semantics for strstreambuf

In libstdc++ the deprecated char* streams are non-copyable, as was
required pre-C++11.

Since C++11 the standard implies that those streams should be copyable,
but doesn't specify the effects of copying them. This is surely a
defect, so for consistency with other implementations this change makes
them movable, but not copyable.

	PR libstdc++/69608
	* include/backward/strstream (strstreambuf): Define move constructor
	and move assignment operator.
	(istrstream, ostrstream, strstream): Likewise.
	* testsuite/backward/strstream_move.cc: New.

From-SVN: r259842