Reapply r345008 "Split MachinePipeliner code into header and cpp files"

Reapply r345008 "Split MachinePipeliner code into header and cpp files"

Split MachinePipeliner code into header and cpp files to allow
inheritance from SwingSchedulerDAG.

This reapplies https://reviews.llvm.org/D56084 after moving the
implementation of the dump functions into the .cpp files. This fixes a
linker error when building with Clang modules enables and local
submodule visibility disabled.

Original patch by Lama Saba <lama.saba@intel.com>!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351077 91177308-0d34-0410-b5e6-96231b3b80d8