Recover instead of crashing on MS assembly when no target is loaded

It's possible to embed the frontend in applications that haven't initialized
backend targets so we need to handle this condition gracefully.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193685 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed