Add a method to get the indentation for an XHPASTNode

Summary: Add a getIndentation() method to XHPASTNode. This method returns the whitespace that was used to indent the node. This is useful for linter rules which enforce various alignment-related nitpicks, such as {D11504} and {D12296}.

Test Plan: See D12296.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12295