Use std::move() to prevent copying.

Bug: http://b/110779387
Test: Build with new toolchain.
Change-Id: I0fa29e9dc2d7f2a30e8e7137d6ff8ba6f03e80b1
diff --git a/cmdline/detail/cmdline_parse_argument_detail.h b/cmdline/detail/cmdline_parse_argument_detail.h
index 2fee27c..65c1114 100644
--- a/cmdline/detail/cmdline_parse_argument_detail.h
+++ b/cmdline/detail/cmdline_parse_argument_detail.h
@@ -443,7 +443,7 @@
 
       assert(!argument_info_.has_range_);
 
-      return result;
+      return std::move(result);
     }
 
     CmdlineParseResult<TArg> result = type_parser.Parse(argument);