Allow specifying multiple locations of the same lint error

Summary:
Some errors (duplicate declaration, invalid number of arguments) have more related places.
We need to notify user if he changes any related place.
This could be currently achieved by triggering errors instead of warnings or by including both files in the range (impossible if the locations are in different files) or by issuing multiple errors.
All options are too aggressive.

Test Plan: Issued error on unmodified line with other location on modified line.

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4392
Format whitespace in XHPAST linter

Test Plan: Linted a method named `f_a`.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4405