Fix pr18235.

The cpp backend is not a reasonable fallback for a missing target. It is a
very special backend, so it is reasonable to use it only if explicitly
requested.

While at it, simplify the interface a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197241 91177308-0d34-0410-b5e6-96231b3b80d8