libstdc++: Add default_sentinel support to stream iterators

Missing pieces of P0896R4 "The One Ranges Proposal" for C++20.

	* include/bits/stream_iterator.h (istream_iterator(default_sentinel_t)):
	Add constructor.
	(operator==(istream_iterator, default_sentinel_t)): Add operator.
	(ostream_iterator::difference_type): Define to ptrdiff_t for C++20.
	* include/bits/streambuf_iterator.h
	(istreambuf_iterator(default_sentinel_t)): Add constructor.
	(operator==(istreambuf_iterator, default_sentinel_t)): Add operator.
	* testsuite/24_iterators/istream_iterator/cons/sentinel.cc:
	New test.
	* testsuite/24_iterators/istream_iterator/sentinel.cc: New test.
	* testsuite/24_iterators/istreambuf_iterator/cons/sentinel.cc:
	New test.
	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc: New test.