[llvm] [flang-rt] Update `test_flang` in CI to use `flang-rt` as one of the projects. (PR #135260)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> So for the part of the script that specifies LLVM_ENABLE_PROJECTS, are you planning to add LLVM_ENABLE_RUNTIMES?

Maybe. I need to figure out a good way to enable good compilation caching when possible while keeping things simple.

> That could be used to build and test compiler-rt and flang-rt as alternative to the standalone build

We would not use the standalone build. We would be using the runtimes build either way. If we did the build separately we would just be using the runtimes build:

```
cmake -GNinja -DCMAKE_BUILD_TYPE=Release -D... ../runtimes
```

That should probably be a configuration `flang-rt` should support if it does not already.

https://github.com/llvm/llvm-project/pull/135260