* include/bits/fs_path.h (path::empty): Add nodiscard attribute.
	* include/bits/range_access.h (empty): Likewise.
	* include/std/string_view (basic_string_view::empty): Likewise.
	* testsuite/21_strings/basic_string_view/capacity/empty_neg.cc: New
	test.
	* testsuite/24_iterators/range_access_cpp17_neg.cc: New test.
	* testsuite/27_io/filesystem/path/query/empty_neg.cc: New test.

From-SVN: r255124