Hotfix for #3289 `ReflectionParameter#getClass()` on `callable` type hint should not raise exceptions

Summary: See #3289 - `ReflectionParameter#getClass()` on `callable` type hint should not raise exceptions

PR is NOT tested as I didn't have time to build HHVM today, so I'll have to trust travis here.

Closes https://github.com/facebook/hhvm/pull/3290

Reviewed By: @ptarjan

Differential Revision: D1460709

Pulled By: svcscm