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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> 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.

You may still compile the runtime separately, using a standalone build, or you don't even need it for the host platform, only for the target platform (cross-compile). In either case, you need a workaround to not add "flang-rt" implicitly. There isn't one for "clang" and it seems nobody requested one.
 
The position
1. "clang" should be added explicitly by the user
2. "mlir" and "flang-rt" should be added implicitly

feels inconcistent to me.

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