[llvm] [Flang] Implicitly add clang if flang enabled. (PR #123964)

[David Truby via llvm-commits]

DavidTruby wrote:

Not sure; I'm not involved personally in any downstream that doesn't use the clang-based driver. I just remember this as the justification previously that we add `mlir` but don't add `clang`.

https://github.com/llvm/llvm-project/pull/123964