[BOLT] Unifying implementations of ext-tsp

After BOLT's merge to LLVM, there are two (almost identical) versions of the
code layout algorithm. The diff unifies the implementations by keeping the one
in LLVM.

There are mild changes in the resulting block orders. I tested the changes
extensively both on the clang binary and on prod services. Didn't see stat sig
differences on average.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D129895