Render internal stack frames more reasonably

Summary: Some stack frames do not have file/line information, e.g. __autoload
triggers. Render these as "Internal".

Test Plan: Reloaded a trace with an internal __autoload() frame, got
"(Internal)" instead of ": 0" with warnings.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 843