libstdc++: Add [[nodiscard]] to iostream members

These are all pure functions and MSVC also marks all of these as
nodiscard except for std::basic_ios::tie() const, but that's been
confirmed as an accidental omission.

libstdc++-v3/ChangeLog:

	* include/bits/basic_ios.h (basic_ios::operator bool()):
	Add [[nodiscard]] attribute.
	(basic_ios::operator!(), basic_ios::rdstate())
	(basic_ios::good(), basic_ios::eof(), basic_ios::fail())
	(basic_ios::bad(), basic_ios::exceptions(), basic_ios::tie())
	(basic_ios::rdbuf(), basic_ios::fill()): Likewise.
	* include/bits/ios_base.h (ios_base::flags()): Likewise.
	(ios_base::precision(), ios_base::width(), ios_base::getloc()):
	Likewise.
	* include/std/fstream (basic_filebuf::is_open)
	(basic_ifstream::rdbuf(), basic_ifstream::is_open)
	(basic_ofstream::rdbuf(), basic_ofstream::is_open)
	(basic_fstream::rdbuf(), basic_fstream::is_open): Likewise.
	* include/std/spanstream (basic_spanbuf::span())
	(basic_ispanstream::span(), basic_ispanstream::rdbuf())
	(basic_ospanstream::span(), basic_ospanstream::rdbuf())
	(basic_spanstream::span(), basic_spanstream::rdbuf()):
	Likewise.
	* include/std/sstream (basic_stringbuf::str())
	(basic_istringstream::rdbuf(), basic_istringstream::str())
	(basic_ostringstream::rdbuf(), basic_ostringstream::str())
	(basic_stringstream::rdbuf(), basic_stringstream::str()):
	Likewise.
	* testsuite/27_io/basic_istream/extractors_arithmetic/char/01.cc:
	Suppress -Wunused-result warnings.
	* testsuite/27_io/basic_istream/extractors_arithmetic/wchar_t/01.cc:
	Likewise.