Check both UNIX- and Windows-style paths from linter output

Summary:
Paths are passed into linters using UNIX-style slashes (/), as returned from the version control system; however,
Filesystem::readablePath swaps them to Windows-style (\) on
Windows when storing the names of the files with lint messages. This causes no lint message's path to match the set of
changed files, and thus no lint warnings are ever produced.

If a lint message's file is not found using the provided filename, also try looking up the UNIX-style filename, on Windows when determining if a lint mesage is "relevant."

Fixes T11248.

Test Plan: Ran arc lint on Windows.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley

Maniphest Tasks: T11248

Differential Revision: https://secure.phabricator.com/D16579