Fix bug in test-release.sh where the script would not exit if any

Fix bug in test-release.sh where the script would not exit if any
of the build stages that are sent through a pipe (e.g. tee) failed.

This potentially allowed builds and/or tests to fail without anyone
noticing. It appears that for the LLVM 3.6.[01] releases this actually
happened for the Ubuntu 14.04LTS binary releases. The essence of the
issue is that without ``set -o pipefail`` the following command in bash
has a zero exit code.

false | tee /dev/null ; exit $?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241599 91177308-0d34-0410-b5e6-96231b3b80d8