Fix the order in which struct fields drop

When a struct implements Drop, its fields should still drop in
declaration order (just as they do when the struct does not implement
Drop).

Fixes #16492.
auto merge of #16493 : kballard/rust/fix_drop_field_order, r=pnkfelix

When a struct implements Drop, its fields should still drop in
declaration order (just as they do when the struct does not implement
Drop).

Fixes #16492.