`for...in` loop doesn't resolve exported identifiers

Input:
```ts
export let foo = '';
for (foo in { x: true }) {
    print(foo);
}
```

Current Result:
```lua
____exports.foo = ""
for ____key in pairs({x = true}) do
    foo = ____key
    print(____exports.foo)
end
```

Expected Result:
```lua
____exports.foo = ""
for ____key in pairs({x = true}) do
    ____exports.foo = ____key
    print(____exports.foo)
end
```