[clang] [clang][test][NFC] Use -fdriver-only over /dev/null (PR #135255)

[Erich Keane via cfe-commits]

https://github.com/erichkeane requested changes to this pull request.

This one shouldn't work. We explicitly don't diagnose this in non-codegen invocations.

https://github.com/llvm/llvm-project/pull/135255