[XRay] A graph Class for the llvm-xray graph

Summary:
In preparation for graph comparison and filtering, this is a library for
representing graphs in LLVM. This will enable easier encapsulation and reuse
of graphs in llvm-xray.

Depends on D28999, D28225

Reviewers: dblaikie, dberris

Reviewed By: dberris

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29005

llvm-svn: 294713