Fix bugpoint execution/reference output file name

sys::fs::unique_file will now loop infinitely if provided with a file name
without '%' characters and the input file already exists. As a result, bugpoint
cannot use a fixed file name for the execution output (including the reference
output).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185166 91177308-0d34-0410-b5e6-96231b3b80d8