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

[David Truby via llvm-commits]

DavidTruby wrote:

That or the cmake should be fixed so that we don't need clang to be in ENABLE_PROJECTS. We don't actually need all of clang, just specific driver stuff that could be built as dependencies in the cmake without requiring clang in its entirety.

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