[Attributor] Add interface to emit remarks in Attributor

Summary:
This patch adds support for the Attributor to emit remarks on behalf of some
other pass. The attributor can now optionally take a callback function that
returns an OptimizationRemarkEmitter object when given a Function pointer. If
this is availible then a remark will be emitted for the corresponding pass
name.

Depends on d1021978

Reviewed By: sstefan1 thegameg

Differential Revision: https://reviews.llvm.org/D102444