[NFC] Move hot cold splitting class to header file

Summary:  This is to facilitate unittests

Reviewers: compnerd, vsk, tejohnson, sebpop, brzycki, SirishP

Reviewed By: tejohnson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68079

llvm-svn: 373151