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.
(cherry picked from commit
ca97c87758d450a946cac31b0db43da0039fabf1)