libstdc++: sstream from string_view (P2495R3) [PR119741]

libstdc++: sstream from string_view (P2495R3) [PR119741]

Add constructors to stringbuf, stringstream, istringstream, and ostringstream,
and a matching overload of str(sv) in each, that take anything convertible to
a string_view in places where the existing ctors and function take a string.
Note this change omits the constraint applied to the istringstream constructor
from string cited as a "drive-by" in P2495R3, as we have determined it is
redundant.

libstdc++-v3/ChangeLog:

	PR libstdc++/119741
	* include/std/sstream: full implementation, really just
	decls, requires clause and plumbing.
	* include/bits/version.def, include/bits/version.h:
	new preprocessor symbol
	__cpp_lib_sstream_from_string_view.
	* testsuite/27_io/basic_stringbuf/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_istringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_ostringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_istringstream/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_ostringstream/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringstream/cons/wchar_t/string_view.cc:
	New tests.