[BOLT] Add option to randomize function split point

For test purposes, we want to split functions at a random split point
to be able to test different layouts without relying on the profile.
This patch introduces an option, that randomly chooses a split point
to partition blocks of a function into hot and cold regions.

Reviewed By: Amir, yota9

Differential Revision: https://reviews.llvm.org/D128773