[BOLT] A new code layout algorithm for function reordering [3b/3]

This is a new algorithm for function layout (reordering) based on the call graph
extracted from a profile data; see diffs down the stack for more details.

This layout is very similar to the existing hfsort+, but perhaps a little better
on some benchmarks. The goals of the change is as follows:

(i) rename and replace hfsort+ with a newer (hopefully better) implementation.
I'd prefer to keep both algs together for some time to simplify evaluation and
transition, but do want to remove hfsort+ once we're confident that there are
no regressions.

(ii) unify the implementation of code layout algorithms across LLVM. Currently
Passes/HfsortPlus.cpp and Utils/CodeLayout.cpp share many implementation-specific
details; this diff unifies the code.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D153039