Ignore PHP types for "needs" symbols

Summary: I maintain a Phabricator library which supports PHP 7, but I can't use PHP 7 types because this causes "unknown class/interface" linter warnings. Ignore PHP built-in types, similar to D18151.

Test Plan:
Ran scripts/phutil_symbols.php on the following file:

<?php

function x(array $a, bool $b, float $f, int $i, string $s, iterable $ii, object $o): void {}

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19590