[Attributor] Clamp operator to extend known state

Summary:
Similar to `^=` operator for IntegerState, this patch introduces a `+=` operator to "clamp" known information.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66635

llvm-svn: 370015