Make sure that there is no case where a signal can occur leaving a partially

Make sure that there is no case where a signal can occur leaving a partially
written output file.  This is important because crashing testcases often write
part of a file out, and the testing harness decides the file is up-to-date next
time the test is run.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2303 91177308-0d34-0410-b5e6-96231b3b80d8