Provide character position information to symbol queries

Summary: Depends on D18937. Ref T13047. When available, provide character positions so external indexers can return more accurate results.

Test Plan: Clicked symbols in Safari, Firefox and Chrome, got sensible-looking character positions.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13047

Differential Revision: https://secure.phabricator.com/D18939