Remove restriction on concatenating Tany with a string

Summary: Looks like an old holdover from when strict mode let you do
basically nothing with `Tany`. It's weird, let's just kill it.

Reviewed By: @elgenie

Differential Revision: D1465255
Tag the SilverStripe framework tests to 3.1.5

Summary: Closes https://github.com/facebook/hhvm/pull/3282

Reviewed By: @fredemmott

Differential Revision: D1465874

Pulled By: svcscm