[clang-format] Fix lambda with ellipsis in return type

Summary:
BTW my actual code that hit this issue is like

```
[a, b = std::move(b)](auto &&... c) mutable -> std::invoke_result_t<decltype(b), decltype(c)...> { /* omitted */ }
```

where this explicit return type is required for SFINAE.

Reviewed By: krasimir, MyDeveloperDay

Patch By: johnchen902

Subscribers: krasimir, dexonsmith, cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D78694