[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:

> The intention of this PR is to prevent building flang-rt as part of the flang build. Rather, build it with LLVM_ENABLE_RUNTIMES. This is how the buildbot is doing.

> As the flang-rt build is similar to compiler-rt, I mimicked compiler-rt in this PR. I am not sure if I missed anything or overdid anything.

That's not how the premerge build system is setup. The list of projects computed here gets sent directly to `monolithic-linux.sh` and `monolithic-windows.sh` which you can see in `generate-buildkite-pipeline-premerge` and `.github/workflows/premerge.yaml`. Those scripts just shove everything into `LLVM_ENABLE_PROJECTS`. We need to migrate `compiler-rt` and `libc` off of the projects build, but haven't done it yet as I need to dig into the host toolchain support story there.

What's the compiler support story like for `flang-rt`? Is it fine to build it with a recent clang or does it need to be built with the just-built compiler? Is it going to have fortran source eventually (or does it already)?

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