A new code layout algorithm for function reordering [1/3]

We are brining a new algorithm for function layout (reordering) based on the
call graph (extracted from a profile data). The algorithm is an improvement of
top of a known heuristic, C^3. It tries to co-locate hot and frequently executed
together functions in the resulting ordering. Unlike C^3, it explores a larger
search space and have an objective closely tied to the performance of
instruction and i-TLB caches. Hence, the name CDS = Cache-Directed Sort.
The algorithm can be used at the linking or post-linking (e.g., BOLT) stage.

This diff modifies the existing data structures to facilitate the implementation
(down the stack). This is a no-op change.

Reviewed By: hoy

Differential Revision: https://reviews.llvm.org/D152833