Add a simple symbol lookup interface for cross-references
Summary: This will get fancier, but here's a basic interface for doing symbol
lookups. Still all pretty tentative.
Test Plan: Looked up various things, got some sensible results.
Reviewers: jungejason, nh, tuomaspelkonen, aran
Reviewed By: tuomaspelkonen
CC: aran, tuomaspelkonen
Differential Revision: 900