Bugfix: lowering of variadic templates of functions

[MaxSagebaum]

Resolves #875

Minimal example: https://cpp2.godbolt.org/z/1bGGanfnr

x: <Ts...: type> type = { 
    func: () = {}
}

Error:

main.cpp2:2:36: error: qualifier contains unexpanded parameter pack 'Ts'
    2 |     template <typename ...Ts> auto x<Ts>::func() -> void{}
      |                                    ^

Correct code:

 template <typename ...Ts> auto x<Ts...>::func() -> void{}

[JohelEGP]

Please, edit the opening comment with "Resolves #875".

[hsutter]

Thanks!