[clang] Reapply "[NFC][clang][Driver] Add tests for --driver-mode=flang" (PR #211724)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

Thanks. This looks good to me. Maybe leave it up for a few more days to see if @MaskRay has any thoughts.

https://github.com/llvm/llvm-project/pull/211724