Don't ask for annotations for arguments of partially-bound functions
Don't ask for annotations for arguments of partially-bound functions

Summary:
In D5248562, the following starts to ask for annotations:

```
function f(x: string): void {}
export default f.bind(null);
```

The missing annotation error is happening because the args of the bound
function are genereated via ArrRestT, which uses an intermediate tvar. This
diff marks the intermediate tvar as "derivable," which is the standard trick,
if hacky.

Reviewed By: gabelevi

Differential Revision: D5290939

fbshipit-source-id: 600718b54d60d984ee9ad6974d8c76afc86e9892

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