[MachineScheduler] Put SchedRegion in an anonymous namespace.

Summary: It pollutes the global namespace otherwise.

Patch by: Bevin Hansson

Reviewers: jonpa

Reviewed By: jonpa

Subscribers: MatzeB, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D37555

llvm-svn: 313148