No need for a wrapping structure for posixTimeToDuration

No need for a wrapping structure for posixTimeToDuration

Summary:
[Folly] No need for a wrapping structure for `posixTimeToDuration`.

We can just use a variant of tag dispatch. In this variant, we pass to `posixTimeToDuration` a default-initialized value of the desired return type and we write overload templates for each possible variant. The argument is used purely for overload resolution and return type deduction, not for its runtime value. It is slightly different from tag dispatch because we do not use separate types which are purely tag types.

Reviewed By: simpkins

Differential Revision: D6371572

fbshipit-source-id: 1987dee31fceec8733caa61495e96489dbf1ca39