R1:4cff4dc68b6e

Interpret search tokens in the for "_..." as substring search

Summary: Ref T13632. Users searching for `__FILE__`, etc., almost certainly mean to perform a substring search.

Test Plan: Added tests and made them pass. Searched for various tokens, saw compiler interpretation in UI.

Maniphest Tasks: T13632

Differential Revision: https://secure.phabricator.com/D21602