[Bugpoint redesign] Output option can now print to STDOUT

Summary:
This also changes all the outs() statements to errs() so the output and
progress streams don't get mixed.

This has been added because D64176 had flaky tests, which I believe were because the reduced file was being catted into `FileCheck`, instead of being pass from STDOUT directly.

Reviewers: chandlerc, dblaikie, xbolva00

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66314

llvm-svn: 369060