Use structs rather than type aliases for Unit::Lift and Unit::Drop

Use structs rather than type aliases for Unit::Lift and Unit::Drop

Summary:
VS 2017 RC brings with it fixes for some things and unfortunately, a few known regressions in expression SFINAE. In this case, alias templates with dependent `decltype()` calls are broken, so switch to a templated struct instead.
It's the very last issue mentioned in: https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/

Reviewed By: yfeldblum

Differential Revision: D4199676

fbshipit-source-id: 9ce157cc891182509a30b000de1b509063387244