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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> How does this work when using a pre-built clang (say for a matching version and/or commit)? We need a way to build flang independently of clang, not least because of resource constraints.

This has already been discussed in this thread. If you don't add `LLVM_ENABLE_PROJECTS=flang`, then `LLVM_ENABLE_PROJECTS=clang` will not be added either. If you have a standalone build of flang, you are not adding  `LLVM_ENABLE_PROJECTS=flang` to the LLVM build.

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