We need to keep the original linkage because it would be illegal to call a shared not-serialized function from a serialized function.
Also, rename the API to create the specialized function.
Generic specialization already takes care of removing metatype arguments of generic functions.
But sometimes non-generic functions have metatype arguments which must be removed.
We need handle this case with a function signature optimization.
This enables, for example, to use `OptionSet` in embedded swift.
rdar://121206953
