PR libstdc++/86447
	* src/c++11/cow-stdexcept.cc [_GLIBCXX_FULLY_DYNAMIC_STRING]
	(logic_error::logic_error(logic_error&&))
	(logic_error::operator=(logic_error&&))
	(runtime_error::runtime_error(runtime_error&&))
	(runtime_error::operator=(runtime_error&&)): Copy strings instead of
	moving, to avoid allocating empty reps for moved-from strings.

From-SVN: r263590