Add tailexpr & -> &mut for generate_mut_trait_impl

[A4-Tacks]

Implement simple & -> &mut on tailexpr for generate_mut_trait_impl

#15581 (comment)

But there should definitely be a rewrite rule for &(...)[...] to &mut (...)[...], and perhaps other common cases. Just because the general problem is impossible doesn't mean the easy cases shouldn't be done; doing the easy cases is what assists are for.

[ShoyuVanilla]

Thanks!