Migrate to llvm::unique_function instead of static member functions for callbacks

A few cleanups suggested in another patch review's comments:

1. Use llvm:unique_function for storing & invoking callbacks from
   Editline to IOHandler
2. Change return type of one of the callback setters from bool to void,
   since it's return value was never used
3. Moved the callback setters inline & made them nonstatic, since that's
   more consistent with other setter definitions
4. Removed the baton parameter since we no longer need it anymore

Differential revision: https://reviews.llvm.org/D50299