Add support for generic lambda with explicit type list

[filipsajdak]

Currently cppfront process the following code:

main: () = {
    g := :<T> () = { v := 0; };
}

And generates:

auto main() -> int{
    auto g {template <typename T> []() -> void{auto v {0}; }};
}

After this change cppfront will generate:

auto main() -> int{
    auto g {[]<typename T>() -> void{auto v {0}; }};
}

Closes: #346. All regression tests pass.

[JohelEGP]

Thank you. This fixes #346.

Code:

g: () = :<T> () = { };
f: () = {
  h := :<T> () = { };   // #346
  :<T> (x: T) -> _ = x; // #346
}
id_fn: () -> _ = :<T> (x: T) -> _ = x; // #346

Generated C++1.

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"


//=== Cpp2 type definitions and function declarations ===========================

#line 1 "x.cpp2"
auto g() -> void;
auto f() -> void;

#line 6 "x.cpp2"
[[nodiscard]] auto id_fn() -> auto;

//=== Cpp2 function definitions =================================================

#line 1 "x.cpp2"
auto g() -> void { []<typename T>() -> void{};  }
auto f() -> void{
  auto h {[]<typename T>() -> void{}}; // #346
  []<typename T>(T const& x) -> auto { return x;  };// #346
}
[[nodiscard]] auto id_fn() -> auto { return []<typename T>(T const& x) -> auto { return x;  };  }// #346

[hsutter]

Sweet! 👏 Thanks @JohelEGP and @filipsajdak . I'm not a big user of explicit template parameter lists on lambdas, so I hadn't noticed yet that that case didn't code gen correctly. Good stuff, appreciated.