`<sstream>`: `basic_stringbuf` shouldn't implement moving with swapping

[StephanTLavavej]

basic_stringbuf implements move construction and move assignment with swap():

STL/stl/inc/sstream

Lines 74 to 88 in 0403d19

	basic_stringbuf(basic_stringbuf&& _Right) : _Mystate(0) {
	_Assign_rv(_STD move(_Right));
	}
	basic_stringbuf& operator=(basic_stringbuf&& _Right) noexcept /* strengthened */ {
	_Assign_rv(_STD move(_Right));
	return *this;
	}
	void _Assign_rv(basic_stringbuf&& _Right) noexcept {
	if (this != _STD addressof(_Right)) {
	_Tidy();
	this->swap(_Right);
	}
	}

This is bogus because swap() activates POCS logic (propagate_on_container_swap), but move assignment needs to activate POCMA logic (propagate_on_container_move_assignment). The unconditional noexcept strengthening is also bogus.

Found by std/input.output/string.streams/stringbuf/stringbuf.cons/move.alloc.pass.cpp in the upcoming libcxx update that I'm working on. The error was sstream(94) : Assertion failed: The allocators of basic_stringbuf should propagate or be equal on swap.