libstdc++: Add <spanstream> declarations to <iosfwd> [PR105284]

libstdc++: Add <spanstream> declarations to <iosfwd> [PR105284]

libstdc++-v3/ChangeLog:

	PR libstdc++/105284
	* include/std/iosfwd: Add declarations for <spanstream> class
	templates and typedefs.
	* include/std/spanstream (basic_spanbuf, basic_ispanstream)
	(basic_ospanstream, basic_spanstream): Remove default template
	arguments.
	* testsuite/27_io/headers/iosfwd/synopsis.cc: Add <spanstream>
	declarations.
	* testsuite/27_io/headers/iosfwd/types.cc: Check <spanstream>
	default arguments.