re PR libstdc++/64476 (std::uninitialized_copy tests assignability the wrong way, resulting in performance pessimization)

	PR libstdc++/64476
	* include/bits/stl_uninitialized.h (uninitialized_copy): Fix
	is_assignable arguments.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/64476.cc:
	New.

From-SVN: r219398