If we're unable to create the TargetMachine, then just quit producing the
backend output; there's no need to report a fatal error.  This reverts r178042.
Part of rdar://13295753 and rdar://13401547


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178102 91177308-0d34-0410-b5e6-96231b3b80d8