libstdc++: Add remaining C++20 additions to <sstream> [P0408R7]

This adds the new overloads of basic_stringbuf::str, and the
corresponding overloads to basic_istringstream, basic_ostringstream and
basic_stringstream.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.21): Tighten patterns.
	(GLIBCXX_3.4.29): Export new symbols.
	* include/bits/alloc_traits.h (__allocator_like): New concept.
	* include/std/sstream (basic_stringbuf::swap): Add exception
	specification.
	(basic_stringbuf::str() const): Add ref-qualifier. Use new
	_M_high_mark function.
	(basic_stringbuf::str(const SAlloc&) const): Define new function.
	(basic_stringbuf::str() &&): Likewise.
	(basic_stringbuf::str(const basic_string<C,T,SAlloc>&)):
	Likewise.
	(basic_stringbuf::str(basic_string<C,T,Alloc>&&)): Likewise.
	(basic_stringbuf::view() const): Use _M_high_mark.
	(basic_istringstream::str, basic_ostringstream::str)
	(basic_stringstream::str): Define new overloads.
	* src/c++20/sstream-inst.cc (basic_stringbuf::str)
	(basic_istringstream::str, basic_ostringstream::str)
	(basic_stringstream::str): Explicit instantiation definitions
	for new overloads.
	* testsuite/27_io/basic_istringstream/view/char/1.cc: Add more
	checks.
	* 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_stringstream/view/char/1.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_istringstream/str/char/2.cc: New test.
	* testsuite/27_io/basic_istringstream/str/wchar_t/2.cc: New test.
	* testsuite/27_io/basic_ostringstream/str/char/3.cc: New test.
	* testsuite/27_io/basic_ostringstream/str/wchar_t/3.cc: New test.
	* testsuite/27_io/basic_stringbuf/str/char/4.cc: New test.
	* testsuite/27_io/basic_stringbuf/str/wchar_t/4.cc: New test.
	* testsuite/27_io/basic_stringstream/str/char/5.cc: New test.
	* testsuite/27_io/basic_stringstream/str/wchar_t/5.cc.cc: New test.