Lots of unittests started failing under asan after r280455. It seems
they've been failing for a long time, but lit silently ignored them.
Downgrade the error so we can figure out what is going on.
Filed http://llvm.org/PR30285.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280674 91177308-0d34-0410-b5e6-96231b3b80d8