swift234

Remember to set AbstractFunctionDecl's generic signature in a few places

I originally added this so that we would keep the signature around
even if type checking failed, and the function was given an
ErrorType.

Add a formal check to the AST verifier for this, and set the signature
in a few places where it wasn't being set.

Note that since we only serialize valid declarations, we don't have
to serialize a reference to the generic signature separately, but we
do have to remember to set it when deserializing, which wasn't being
done for destructors.