libstdc++: Define std::basic_stringbuf::view() for old std::string ABI

libstdc++: Define std::basic_stringbuf::view() for old std::string ABI

Unlike the new str()&& members in <sstream>, there is no real difficulty
in supporting the new view() members for the old std::string ABI.
Enabling it fixes errors in <chrono> where std::ostringstream::view() is
used by ostream insertion operators for calendar types.

We just need to use [[gnu::always_inline]] on the view() members for the
old ABI, because the library doesn't contain instantiations of them for
the old ABI. Making them always inline avoids needing to add those
instantiations and export them.

libstdc++-v3/ChangeLog:

	* include/std/sstream  (basic_stringbuf::view): Define for old
	std::string ABI.
	(basic_istringstream::view, basic_0stringstream::view)
	(basic_stringstream::view): Likewise.
	* testsuite/27_io/basic_istringstream/view/char/1.cc: Remove
	{ dg-require-effective-target cxx11_abi }.
	* testsuite/27_io/basic_istringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/view/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/view/wchar_t/1.cc:
	Likewise.