[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:

> I start to think the proper fix probably should be that we stop building flang-rt as part of the flang project in cmake, and uses standalone runtime premerge build when testing flang or flang-rt changes.

That sounds about right. We need to rework the runtimes build for premerge anyways which has been on my TODO list for a while and something I hope to get too soon.

What are the compiler requirements for flang-rt? From what I can tell it is entirely c++ currently. Does it need ToT clang/flang?

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