LWG-1203 More useful rvalue stream insertion

[StephanTLavavej]

LWG-1203 More useful rvalue stream insertion

This was mostly implemented in VS 2017 15.8. However, we used is_base_of which doesn't match the "publicly and unambiguously derived" wording in LWG-1203. (See [meta.rel]: "[Note: Base classes that are private, protected, or ambiguous are, nonetheless, base classes.—end note]")

STL/stl/inc/istream

Lines 879 to 886 in e034296

	template <class _Istr, class _Ty,
	enable_if_t<conjunction_v< // avoid infinite recursion
	negation<is_lvalue_reference<_Istr>>, is_base_of<ios_base, _Istr>, _Can_stream_in<_Istr, _Ty>>,
	int> = 0>
	_Istr&& operator>>(_Istr&& _Is, _Ty&& _Val) { // extract from rvalue stream
	_Is >> _STD forward<_Ty>(_Val);
	return _STD move(_Is);
	}

STL/stl/inc/ostream

Lines 969 to 976 in e034296

	template <class _Ostr, class _Ty,
	enable_if_t<conjunction_v< // prevent infinite recursion
	negation<is_lvalue_reference<_Ostr>>, is_base_of<ios_base, _Ostr>, _Can_stream_out<_Ostr, _Ty>>,
	int> = 0>
	_Ostr&& operator<<(_Ostr&& _Os, const _Ty& _Val) { // insert to rvalue stream
	_Os << _Val;
	return _STD move(_Os);
	}