Avoid crashing when failing to emit a thunk

If we crash, we raise a crash handler dialog, and that's really
annoying.  Even though we can't emit correct IR until we have musttail,
don't crash.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205948 91177308-0d34-0410-b5e6-96231b3b80d8