Resubmit rL345008 "Split MachinePipeliner code into header and cpp files"

Resubmitted in rL345290 and reverted in rL350345 due to failures in
http://green.lab.llvm.org/green/job/lldb-cmake/
Resubmitting after a workaround to lldb-cmake failure was
committed in rL350346, more info in https://reviews.llvm.org/D56084

llvm-svn: 350493