Regression tests now using pipefail

I just committed a change (r187261) to use pipefail when running the
regression tests. This means that tests
that use pipes like

opt .... | FileCheck

will now fail if opt fails. This would have avoid some test bitrot in
the past. For example, we had
tests doing

opt -S ... | not grep ...

and they were still passing even after the the test itself was not
even passing the verifier.

If you have out of tree tests that depend on the old behavior, don't
worry, it is still available. Just add

config.pipefail = False

to the lit.local.cfg of the corresponding directory.