Actually mark Unexpected as cold

Actually mark Unexpected as cold

Summary:
Testing indicates that GCC ignores the cold attribute when the function
is available for inlining. Because Unexpected is a template class we
can't make the constructors non-inline, but we can make it derive from a
class with a cold constructor, which has the effect of making all the
Unexpected constructors implicitly cold.

Reviewed By: yfeldblum

Differential Revision: D6261013

fbshipit-source-id: 482e49253d5b104742018133c53fb60279dd9f9b