Generation failed when using LuaMultiReturn in type assertion

see [playground example](https://typescripttolua.github.io/play/#code/DYUwLgBAZgrgdgYwFwQIZwJ4CgsApaJoDOEuYAFgJZEoBuA9pQCYA0EAdJ6gE4DmNaTAEoIAXgB8EADIxUAWRjAwlAErgY3OAB4A2kTDdKcXm32HjAXXFDcARjYAmNgGYhQA)

The code below will cause an error in generation.
```typescript
let func: any
let [a, b] = (func as (this: void, ...args: any) => LuaMultiReturn<[string, string]>)(1, 2, 3)
```