PR libstdc++/54388
	* include/std/array (array::at() const): Ensure lvalue result.
	* testsuite/23_containers/array/element_access/54388.cc: New.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc: Adjust
	dg-error line numbers.
	* testsuite/23_containers/array/tuple_interface/tuple_element_neg.cc:
	Likewise.

From-SVN: r191114