Allow setCustomSeverityRules to be overridden in subclasses
I am writing a proxy linter that can be used to wrap any ArcanistExternalLinter and execute all commands within a Docker container (see ArcanistDockerContainerLinterProxy from 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
Differential Revision: https://secure.phabricator.com/D20514