[clang][CFG] Support construction of a weak topological ordering of the CFG.

This patch adds support for building a weak topological ordering (WTO) of the
CFG blocks, based on a limit flow graph constructed via (repeated) interval
partitioning of the CFG.

This patch is part 2 of 2 for adding WTO support.

Differential Revision: https://reviews.llvm.org/D153058