Speculative fix for Windows buildbot after r209138

It appears that Windows doesn't like renaming over open files, which we
do in clearOutputFiles. The file being compiled should be safe to
removed, but this isn't very satisfying - we don't want to manually
manage the lifetime of files we cannot prove have no references.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209195 91177308-0d34-0410-b5e6-96231b3b80d8