Lint dynamic string usage in parameters treated as safe

Summary:
This disallows code like this:

$cmd = 'ls';
execx($cmd);

But I guess it's not that big deal?

Test Plan: Linted whole Arcanist and Phabricator codebases, most parts looks fixable.

Reviewers: epriestley

CC: nh, aran, Korvin

Differential Revision: https://secure.phabricator.com/D4794