Summary: `folly::cold_detail::ColdClass` was marking things (like `folly::Unexpected`) cold, but at the cost of inhibiting the inliner from doing its job. This is leading to bad codegen, which offsets any small wins we mind get for the `cold` attribute.

Reviewed By: yfeldblum

Differential Revision: D19324159

fbshipit-source-id: 7ed431b6c9d6e963c3bf438c707fa6cf6a38bf9d