[clang] [NFC][clang][Driver] Add tests for --driver-mode=flang (PR #207658)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> Why do we need clang/test/Driver/flang/ when we have flang/test/Driver/ ?

The tests there are for `clang --driver-mode=flang`. Since they explicitly use the `clang` driver, I think it makes more sense for them to be there. We could change the directory name, or add a README.md there if you think that better.

https://github.com/llvm/llvm-project/pull/207658