Allow paren-wrapped function literals with `immed`

@rwaldron I initially included the test case in a separate file dedicated to this issue in the `tests/unit/fixtures/` directory, but the current placement seems a little more discoverable/less cluttered. Let me know if/where you'd like me to move it!

This should resolve gh-1737

Commit message:

> Function literals must be wrapped in parenthesis if their properties are
> to be accessed. Ensure that this syntax does not trigger warnings when
> the `immed` option is set.
> 
> The native Function prototype may be extended with any arbitrary
> property, making the current property whitelisting approach
> inappropriate.