[BOLT] Remove old layout from function layout

To track whether a function's new layout is different from its old
layout when updating it, the old layout would be kept around in memory
indefinitely (if the new layout is different). This was used only for
debugging/logging purposes. This patch forces the caller of function
layout's update method to copy the old layout into a temporary if they
need it by removing the old layout fields.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D131413