Use pipefail when available.

This change makes test with RUN lines like
RUN: opt ... | FileCheck

fail if opt fails, even if it prints what FileCheck wants. Enabling this
found some interesting cases of broken tests that were not being noticed
because opt (or some other tool) was crashing late.

Pipefail is used when the shell supports it or when using the internal
python based tester.

llvm-svn: 187261