[Attributor] Introduce bit-encodings for abstract states

Summary:
The IntegerState, and its sepecialization BooleanState, can be used to
simplify the implementation of abstract attributes. The two abstract
state implementations provide storage and helpers to deal with bit-wise
encoded state.

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60012

llvm-svn: 363407