Allow AAST to extract string literal values from HEREDOCs
Ref T13577. I'd like to arc lint --everything to find other bad calls to pht() and similar functions, but n_HEREDOC nodes currently can not generate a response to "getStringLiteralValue()".
Support literal extraction from heredocs.
Test Plan: Added a test, made it pass. Will lint everything.
Maniphest Tasks: T13577
Differential Revision: https://secure.phabricator.com/D21455