Allow setCustomSeverityRules to be overridden in subclasses

Summary:
I am writing a proxy linter that can be used to wrap any ArcanistExternalLinter and execute all commands within a Docker container (see [[https://github.com/freelancer/flarc/blob/master/src/lint/linter/ArcanistDockerContainerLinterProxy.php |ArcanistDockerContainerLinterProxy]] from [[https://github.com/freelancer/flarc | flarc]]). In order for ArcanistDockerContainerLinterProxy to behave like the ArcanistExternalLinter that is being proxied, final needs to be removed from some methods.

I figured this was reasonable to submit upstream as a similar change (D19630: Allow `willLintPaths` and `didLintPaths` to be overridden) was previously accepted.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20514