libstdc++: Make istreambuf_iterator base class consistent (PR92285)

libstdc++: Make istreambuf_iterator base class consistent (PR92285)

Since LWG 445 was implemented for GCC 4.7, the std::iterator base class
of std::istreambuf_iterator changes type depending on the -std mode
used. This creates an ABI incompatibility between different -std modes.

This change ensures the base class always has the same type. This makes
layout for C++98 compatible with the current -std=gnu++14 default, but
no longer compatible with C++98 code from previous releases. In practice
this is unlikely to cause real problems, because it only affects the
layout of types with two std::iterator base classes, one of which comes
from std::istreambuf_iterator. Such types are expected to be vanishingly
rare.

	PR libstdc++/92285
	* include/bits/streambuf_iterator.h (istreambuf_iterator): Make type
	of base class independent of __cplusplus value.
	[__cplusplus < 201103L] (istreambuf_iterator::reference): Override the
	type defined in the base class
	* testsuite/24_iterators/istreambuf_iterator/92285.cc: New test.
	* testsuite/24_iterators/istreambuf_iterator/requirements/
	base_classes.cc: Adjust expected base class for C++98.

From-SVN: r280116