[XRAY] A Color Choosing helper for XRay Graph

Summary:
In Preparation for graph comparison, this patch breaks out the color
choice code from xray-graph into a library and adds polynomials for
the Sequential and Difference sets from ColorBrewer.

Depends on D29005

Reviewers: dblaikie, chandlerc, dberris

Reviewed By: dberris

Subscribers: chandlerc, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D29363

llvm-svn: 296210