Keywords `auto` and `ref` must be adjacent for `auto ref` return.

Similar to `auto ref` parameters in 2.111, it's now deprecated to declare an `auto ref` return type without putting those two keywords next to each other as well.

---
ref auto int f() => 3;
auto { ref int g() => 3; }

// Correction:
auto ref f() => 3;
auto ref g() => 3;
---
