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

[David Truby via llvm-commits]

DavidTruby wrote:

I wouldn't mind this but I think some downstreams don't depend on the clang-based driver so might have objections.

The difference between this/flang-rt and e.g. clang not adding other things is that clang can use alternatives for all of its dependencies, and does by default on most platforms. There's no alternative runtime to `flang` and it's not designed to support that in future; the runtime is considered a part of the compiler that must always be in sync with it.

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