Improve performacne of PhutilSymbolLoader

Summary: Currently, we build way more symbols than we need to when doing `loadObjects()`. Instead, select only the candidates we know will match the request.

Test Plan: 650ms -> 380ms on Differential revision page, which hits this more than it probably should.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7584