Introduce a CallGraph updater helper class

The CallGraphUpdater is a helper that simplifies the process of updating
the call graph, both old and new style, while running an CGSCC pass.

The uses are contained in different commits, e.g. D70767.

More functionality is added as we need it.

Reviewed By: modocache, hfinkel

Differential Revision: https://reviews.llvm.org/D70927