Allow paren-wrapped function literals with `immed`

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.

Closes gh-1737
Merge pull request #1843 from jugglinmike/1737-immed-expr

Allow paren-wrapped function literals with `immed`