Mark constructing an Unexpected as cold

Summary:
By marking the main constructors of `Unexpected` cold we are telling the compiler that we don't expect to execute them often. This can improve the locality and performance of the `Expected` branch, and reduce the code-size for the `Unexpected` branch.

The constructors we have marked cold here are only the ones used to originate an `Unexpected`. We don't mark the copy- move- or conversion-constructors as `cold` because they will typically be used in error-handling branches and it is not correct to say that they are colder than their surroundings in that case. Note that the entire error-handling branch is likely to be deduced to be cold because we told the compiler that it is likely that hasValue() is true and hasError() is false.

Because `makeUnexpected` unconditionally invokes one of these cold constructors it will inherit the property of being cold.

The GCC function attribute reference, which describes what `cold` means, is here: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

Reviewed By: yfeldblum

Differential Revision: D6234305

fbshipit-source-id: 6876073e92ce54936ba7152175d76be653f5f463