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

[Tarun Prabhu via cfe-commits]

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tarunprabhu wrote:

I am not sure if an exhaustive check of all platforms is strictly necessary here, but I don't have a strong opinion on this, so I am ok with leaving it as it is for now.

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