Fix Signature rewriting
Fix Signature rewriting

Summary:
{D4019174} broke the rewriting of Signature annotations because we
would return the original class name instead of the thing it mapped to...

Reviewed By: dariorussi

Differential Revision: D4039164

fbshipit-source-id: bac27ee967d3387445855250baa648bdf24304c0

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex